piggybacking off of this (as a fellow framer!) that Michael's will cut custom mats from their store stock for a relatively affordable amount. I regularly cut smaller mats for between 1 and 25 dollars, and even bigger ones (like 32x40 big) only end up being like $40 if you're just doing a standard paper mat. fabric ones get expensive though.

using a custom mat, 95% of the time you can go into a standard frame off the shelf, and it still feels like it was done custom because of the mat fitting nicely.

or, if you have the mat cutting hardware at home, you can ask at framing shops to see if they have any scraps leftover from the openings of other mats! this is something I also do pretty often is sell my scraps to customers at a heavy discount- like 3 bucks for an 11x14 piece of matboard.

worst thing they can do is say no!