When I have adopted my cats from sanctuaries, there is always a
condition that only spayed pets will be put out for adoption.
The sanctuary (usually a charity) funds the spaying of pets before
they are adopted (if not alreayd done), and then if you offer a
contribution to the "cost" of the pets, they accept gratefully.  

In the UK the spaying of a female cat can cost up to $300 (equivalent)
so a contribution of, say, $100 (equiv) is nothing, to obtain a
healthy pet who will be a loving friend for many years.