Is there anything I can feed or spray on a stray cat to keep the bugs off of her.?

A stray cat living in our woods had kittens. A few days after she still had a messy butt, but we assumed it was because of having the kittens, but it has been weeks and she still over grooms her butt and legs. I think it is because of all of the bugs on her, and then I think the sores on her butt and legs are attracting the bugs even more. I cannot catch her, and I cannot trap her because she is too smart to ever go on our porch even when bribed with food. Is there anything I can feed her or spray on her to deter the bugs. We can get close to her, but not knowingly touch her. I have only touched her back once when she wasn't looking because she thought I was her kitten behind her. She looks miserable so I am desperate for any answers you may have. If I ask the vet they will obviously say that she needs flea medicine, but they don't sell them unless they see the animal, which is not an option.