Same problem Valve Stems striped
I had the same problem with the Valve Stems being stripped in a rental that I have just remodeled, with plumbing of 70 years old and the valves were at lest 40 years old if not the originals. This probably is the worst case scenario I am about to tell you.
I to, did not know the brand of the Valve Stems. Your only choice is to remove one of the Valve Stems with an appropriate wrench, or what MG said "a faucet socket" and find the oldest Pluming Supply store around to see if they have the parts. You will not find these pieces at Home Depot if they were like mine, which looked like and weighed about the same as 30mm cannon shells. You could be lucky and the former owner had plumbing work done to the shower in the last 20 years and the valve stems should be a snap to find and they will not be as big or heavy.
The supply store I went to, did have the Stems but no handles. Do not worry if this happens to you. I just asked every plumbing supply store I ran across if they had hardware for the new stems. I found one place relatively quickly. Remember if the store has old stock they are looking to get ride of it, since most plumbers (at least in my area) would have just given me new Valves. The suppliers can’t sell older materials easily.
That is a second option, replace the valves entirely and that brings on a whole new set of expenses.
Oh, the plumbing supply store will know the make of the valve stem when they see it, if they know anything about plumbing.
I hope this helps.