What is a good non-toxic flea treatment for geriatric cats?

I have two white cats that are 14 years old, brother and sister, and I have had them since they were 6 weeks old. They are indoor cats and the only time they are outdoors is when they go to the vet in their carriers. They have developed a flea problem and a skin allergy from the fleas. I dont want to use a toxic flea spray on them especially the male since his skin is more sensitive. The vet gave me frontline but it had a lot of pesticides and worm medicine in it that they absorbed thru the skin. They werent supposed to have any kind of reaction but they both had very bad reactions both physically and mentally and it took several months for them to return to normal. They have developed fleas again but I dont know how since they are never outside and again the vet wants to put a spray on them that contains pyrethins which I am very leary of doing. Can someone tell me if there is an herbal or natural spray or shampoo out there without pesticides that I could use on them? I have been shampooing them every week in an Oatmeal shampoo with aloe vera in it and putting hydrocortisone cream on the worst places which helps with the itching. My vet assures me that there will be no adverse effects with the flea spray and the shampoo but I just have a bad feeling about them. The brand names are Mal-A-Ket Shampoo and Ovitrol Plus Flea, Tick & Bot Spray for Dogs, Cats & Horses. The reviews say that the fumes can adversely affect adult cats from the spray so why would you use it on them and to use it in an open environment. You cant do that if they are indoor cats. Hope someone knows of something else I can use on them that will be non-toxic and hypoallergenic for their skins. Thanking in advance for your replies.