Buy Dominion L from eBay for under $30, a termiticide, which has as its active ingredient Imidacloprid. This is the same active ingredient as Advantage.
You need some oral syringes of at least 5-10cc size.
Draw 1cc Dominion L into the syringe and then approximately 4-6 cc of warm water. Shake well and apply to skin just like Advantage.
The 28oz bottle will last a long time. Treat bi-weekly for 8 weeks or so if you have a pretty serious problem, then monthly.
Remember, fleas have a life cycle and your pet becomes the tool to kill them. Your pet moving in his environment causes flea eggs to hatch. It will take a little time to break the cycle. Fleas die within hours of contact.
It must be on skin, run the syringe tip against the fur and make sure to get it to skin. Rub it in, rub some on the belly. It is very safe and works like gangbusters.
People who have lots of hunting dogs cannot afford flea meds for them all, my old vet is who told me about this a long time ago. Everyone who I tell and who then does it comes back thanking me.
I use on dogs and cats alike. We were also at the end of our rope when we were told and we have never had any issue since.
Imidacloprid is systemic, it works by getting in the skin. It also is a fantastic treatment for areas outdoors and in. Read the label, mix and spray it where the issues are.
Good luck.