If you don't have enough money to buy a $10 can of formula, how are you going to pay for the shots, the spay, the microchip, and other vet bills? Maybe you shouldn't have gotten a pet.
Also the woman shouldn't have given them away until they were 8wks. old, minimum.
At 8wks. they can be switched to wet food. Try giving him a can of wet cat food. When he reaches 9-10 weeks you can start mixing wet with dry food and by 12 weeks switch him to dry food.
Once the cat is adult, I suggest leaving dry food out 24/7 and giving them a few spoonfuls of wet food each morning and each evening. (or just once a day if your cat starts to gain weight too fast.)
And the cat knows what the water is...she just wants mom's milk. :)
Please take care of the little girl:)