They make "kits" where you but each side in the correct dimension and you have to get the backer and acrylic/glass cut to size (Michaels) - if you buy a cheap frame larger than you need that has acrylic you can easily cut it to size. Michael's has a 40% off one item coupon all the time, use it for your cheap frame to pull the acrylic and backer from. I used to work at one and that was a waaaayyy cheaper alternative to a custom frame. The other option is to buy a matt - they aren't too expensive - and go up in frame size.
I'm Mike from Mike's Toys! That exhibited at Power-Con the last three years BTW.