Good I am glad you will do that!! This is a really great thing to do and believe me, shelters will LOVE you for helping them out. You could call them now and say you are interested in fostering pregnant cats in the future and would like to know if they do it and what you need to know first.
BTW, if you were to just let the cat outside to find a mate, you have to think about the consiquences. Cats are not people, they cannot tell who is a good or bad mate and the male cats do not take no for an answer. You female could get very hurt in the process especially since females CAN get pregnant as young as 4-5 months (which would be like a young child having a baby!) but it doesn't mean that they are ready or mature enough to handle it. Males can mature later and you do not want your baby hurt by a rough, agressive male tom cat. Plus you never know what diseases he may have. He could have FIV or FeLV or a number of other diseases. You also do not know what diseases and problems run in the genetic history of the particular cats, yours or the male. The cats may appear healthy and be deemed healthy by a vet, but unless you know the genetic history, you are taking a great risk allowing them to breed without knowing.
Just something to think about, we don't just telll you not to let your cats out because of the overpopulation or because we are some crazy cat people
There are a lot of health reasons that many people do not even realize. This is part of the reason why BYB is such a bad thing too.