First of all I would seek help getting them spayed and neutered. At the same time a flea treatment can be done. But battling fleas while the cats are reproducing is putting the cart before the horse.
Ask your shelter if they can help with TNR, or RTF (return to field), check with all local animal organizations, Alley Cat Allies Feral Friends network, and FixFinder. If nothing close, expand your search. To get a colony of cats fixed it might pay off to drive somewhere not so close.
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