I had the same problem as you. Dog had seizures, allergies and medical problems that if he even got near a chemical he'd seize.
After searching and searching I finally found 'where' to buy real natural products that work and were safe for my dog.
Find a real fancy high priced natural organic pet food store near you. I say this because they are the only ones to carry 'actual' safe products that you will need to find for your pet.
The owners of these stores really care and they are very knowledgeable. Especially about pets with medical issues.
They will have natural flea control products that are safe and they will know which ones work. I bought several spray products to keep the fleas at bay and several natural flea shampoos.
Because of the new labeling laws natural products are not allowed to use the words "Kills Fleas", even though they do.
You will not find these products in any box store or on line.
Good Luck a little leg work on your part and you will find the products that are safe for your pet. :C