Spacers are similar to fillets so try searching that too, or why not ask a framers to order you some.
It sounds complicated though, and wouldn't it be better to leave it to a framer?
I found this advice when I did a quick search, it will probably put you off doing it yourself.

fillets
These are paper-covered slips of wood used to hold the glass at the front of the moulding, to create a box frame - allowing a space of approx 10-40 mm between the glass and the artwork. This is used both for textured or 3-D artwork, and often for work of value where conservation is a priority.

frame spacers
Similar to fillets, frame spacers are made of transparent plastic and are stuck to the inside of the glass, hidden under the moulding rebate, again to prevent the glass touching the artwork. This is recommended where there is no window mount to perform this function. Frame spacers come in several depths, from 1.5mm to 5 mm.


Question:
Could you please advise the best way to fit fillets into mats? In fact, any helpful information how to cut, measure and use fillets.
Brian's Answer:
Fillets!
1. Measure the sight size of the image.
2. Carefully measure the visible width of the fillet.
3. Draw a picture, do the arithmetic, and figure out the size of the mat opening you require to yield the proper sight size once the fillet is installed.
4. Cut the mat with a reverse bevel. This is better than a straight cut mat because the leading point of the bevel will crush ever so slightly, giving you a tight fit.
5. Mitre all four pieces of the fillet. Cut them about 2mm longer than you know you need them.
Make sure the outside visible edge of the fillet will be the correct size to match against the edge of the mat.
6. Shave them down little by little with the Morso until each side fits exactly. Test the fillet pieces against the side of the mat as you trim and label each piece with its corresponding side of the mat.
It is crucial to have a Morso or a proper fillet trimmer. I have done this by trimming the mat instead of the fillet and it works but it is clumsy and inefficient.
7. Glue the fillet together. Some framers don't glue at all. Some framers ATG each fillet strip into the side of the mat and glue each fillet corner as they go. Do what works best for you, but whenever possible, I like to begin the installation with the fillet glued together.
Installation:
1.Glue the fillet into the mat with numerous dots of Elmer's Glue ( PVA ), not a continuous bead of glue. I worry that too much moisture may warp the matboard. I also worry that too much glue will ooze out.
I have used ATG instead of glue but if the fillet warps even slightly away from the mat, ATG will not hold it tight for long. Glue will.
2. Clamp the fillet onto the mat using banker's clips. Check an office supply store. Clamp every 5 or 6cm and use matboard scraps to keep the clips from scratching the mat.
Let it dry a couple of hours.
3. Build up the back level with the flange of the fillet. Different fillets have different flange thicknesses, so experiment. Typically two layers of matboard or 6-ply rag will be pretty close.
Cut strips wide enough to build up the entire width of the mat and ATG them into place.
Other considerations:
1. Conservation. Some framers cut a rag mat to insulate the fillet from the art. Some let a few millimeters show, some hide it a few millimeters back from the fillet. Some framers consider the wood and varnish of the fillet as unacceptable items inside a conservation package.
2. A picture is worth a thousand words, so check out Vivian Kistler's Library of Professional Picture Framing Volume 2, page 89. It should clear up most of your questions.
Hope this is useful. There are some choices here. You will do fine. Fillets are beautiful but they will test your skill. Have fun!

I took mine to a framers for spacers to be added. all it is is a piece of mount board cut out to the inside edge of the frame. 50x70 = £12

it is very hard to cut them yourself (i've tried) especially if the frame is thin.

if you are talking deep spacers like a box frame then i would definatly see a framer.

i've come up with a soloution. got some foam stripping (double sided foamed tape) going to stick one side to the inside of the frame and then seal it with acid free archival tape before putting the print on.

This will mean that the print is away from the glass and only in contact with the acid free tape.

ok, so that didnt work, at least not on the neilsen frames ive got as the resess is just too damn thin!

might work on a real chuncky frame with a resess of about 10mm!

perhaps my A0 JD frame will allow me to test it out!

Lion framing supplies have them in and I nearly bought some but used strips of moutboard to space instead. Try to use the hayaku tape if you can, it's not as difficult as you might think and is meant to be better for your artwork.

I posted my first float mount frames on here in the week but should probably post them again in the framing section. Making some more frames today for my Vandals and Goldie pieces.

Hello all
I've a couple of prints booked in for framing very soon. I'm thinking of getting a standard white box frame on both with coloured spacers to reflect the main colour on each print. I've never had coloured spacers before allways stick with white on white or black on black. Has anyone any photos or advice as to whether it looks good. Or does it generally detract from the image. 


Thoughts? 

can't go wrong with classic white imo, the rest you may end up reframing when you grow out of it

Not for me. Funky spacers and framing might look cute, but personally the perfect framing is the one that you forget about - the one that goes unnoticed and let the art speak for itself. And size wise, the smaller the better - wall space is a scarce resources, so I frame just large enough that no shade falls on the art, but not bigger.
But as usual, there isn’t a right way. If you find that the coloured spacers add to the image presentation for you, go for it!

All my pictures are in either white or black frames. Just feel like a change. It's a full conservation frame job  on both pieces not cheap so I want to get it right first time. 

a good framer doesn't necessarily have a good eye for mixing up colour. They might be, but its not something everyone can do. You could always mix the colour you want and give that to the framer to use. The advantage of taking the bull by the horns and doing it yourself means you'll be able to change them any time you wanted. Use (cheap) wood spacers and paint with acrylics, that's what I find best.

ha. Yep that's right, sometimes I've had to re do my framing a number of times just to get rid of a tiny speck or hair. I'm a printmaker so have a very keen eye for detail and not a lot gets past me which can be a burden at times. There are some fancy bits of kit you can buy like mini hoovers for this, I'll get one someday.

I say let the spacers do the talking.

You have used coloured mounts. The OP is asking about coloured spacers. 

On another note, does PFG ask permission before posting up the framed pieces on IG?

im torn. Maybe do the eelus and leave the other one simple.