Well, there are slotted cleats that will bolt into your clipless shoes. Be aware that modern bike shoes tend to have a much larger toe box than traditional bike shoes, so you may need to use off-road toeclips for them to fit.

Otherwise there are traditional bike shoes either used, NOS, or new production out there as well.

If you're not planning to use slotted cleats, you could look at some of the bike shoes marketed to modern fixed gear riders from e.g. Vans and such like.