You can buy the embroidery rings at any craft, fabric store...even WalMart. I have actually made custom socks and they work very well.
The question I have is:
If you are making custom socks, why not build a square frame out of pvc and make one big 11"x5.5"x14" sock? Make more than one....make them with stainless snaps and cut the material to have flaps that go around the pvc frame.
.....or....buy brass or stainless eyelets in the top (folded over double for strength) and use zipties or stainless hooks....or plastic shower curtain rings....