Pre-B slide stops are hard to come by. Your best best will be to get one for a 75B
as you continue your search.
One place where I've found a couple of pre-B parts, from time to time, is Jack First.
www.jackfirstgun.com. Picked up a pre-B hammer there when I couldn't find one anywhere else.
You might check with CZ-USA (call them, don't try email.)
They MAY have some, or some of the stops used on the CZ-75B Retro, which are very similar (if not identical.)
[Corrected the bad link, above. Sorry.]