I'd be looking for a new vet group to handle the fosters.
The rescue group that helped me with Cindy actually had their own test kits for FIV/FELV. Might be something to look into getting some. I know they only charged me like $10-$15, at vet's is like $35/$40. I'm sure they were buying these kits in bulk. That way all the new fosters coming in you'd know right they were tested and the results.