Start hanging out at Goodwill, Deseret Industries, Salvation Army, and such thrift stores. About 3/4 of the frames in my house came from such places and none cost over about $5.00.
Chuck the glass and replace the back with 3/8" - 1/2" plywood and cover it with felt (bright red works well, blue doesn't) from the scrap bins at a sewing store. Staple it to the plywood and use brass"L" screw-in hooks to hang the knives.
OR
use square iron nails from Rockler's and tie the knives in place with 5# test monofilament through the back, around the knife just forward of the guard, and back through the plywood.
I am in the process of remounting some of my little belly guns that way on 'flag red' felt from Joann's. (I scrapped the infantry blue cloth that I used originally.) The nickel and blued parts vanished against it and on lt the brass and some of the grips showed up.
I'll try to get some pics to show you.