I really commend you for what you are doing, but like nancyjac says the feral population will continue to grow and eventually it will get too large for you to manage by yourself. These cats also need vaccinations for feline leukemia and rabies. As far as the food issue, I do know that Walmart used to offer free broken bags of cat foods, as well as dog food, to rescue groups and I bet a lot of grocery stores do too. No harm in asking. Good luck, and thank you.
Definitely check with local shelters and cat rescue groups in in your area. Many of them may be able to help you with food that has been donated to them or at least point you in the direction of organizations that can. Even more important, most of them are or are associated with groups that do trap, spay/neuter and release of feral cats (typically called TNR). What you are doing is wonderful, but unless it includes TNR, the number of cats you are feeding will continue to grow. You might also consider teaming up with a shelter or rescue group. Your experience with feral cats would be a valuable asset to them and in turn they could provide you with food and supplies for these cats.
Best of luck to you and thank you for what you do.
In our area, there is a charity that does free spaying and neutering of feral cats, it's that trap-spay/neuter-release-monitor kind of program. If you have something like that in your area, reach out to them. It would keep the population from getting bigger, and there might be resources for food too. Also try the ASPCA and the Humane Society.