If your cats had fleas then the bad news is your house will. The fleas lay eggs - lots & lots of eggs - that drop off, turn into larvae, pupate, and emerge when they get indications of something to feed on being nearby.
Did the vet check to see if the cats did have fleas? You use a flea comb with the cat stood on white paper, and then moisten any dark bits you comb out. If they stain the paper pink-ish they are flea dirt - flea poo.
You can buy Advantage online without a prescription and it's effective - there is a question mark over Frontline.
The RIP Fleas website explains clearly about the flea lifecycle and why just treating the cats won't clear them. You need to treat the whole house with RIP Fleas, Acclaim, Indorex or a similar spray product. Flea 'bombs' are not effective - they don't penetrate the dark spaces the larvae retreat into. You MUST follow the instructions, if you do the products are safe & effective.