Products for cats that kill fleas?

Question

I thought I had gotten rid of the fleas on my cat but surprise I saw flea dirt and combed her and found two or three this morning. I had used sentry flea powder and also sentry spot on but that doesn't seem to be working. I will have to wait atleast two more weeks to use a new product. What would you suggest??? I know most products are pretty expensive but I want to make sure the work and don't have side effects.