Kathy asked a follow-up that was a little more generic, and I think it makes a good discussion.
I was looking online and found bulk supply of Frontline Plus on Ebay. It says that the formulation is the same for cats and dogs except that for cats it's a bit stronger... Is this true? This would be the only affordable way for me to treat the boys right now. It would truely help out with finances. The flea problem is awful currently. We live on a barranca and have all sorts of fun critters visit the yard to share hitch-hikers with our 'indoor only' boys.
Frontline Plus is manufactured by Merial, and is a common topical flea and tick prevention, though not my personal preference. Based on my experience, published data, and discussions with well-recognized parasitology specialists, I much prefer the Advantage line of products by Bayer or Vectra by Summit (sold under the brand FirstShield in Banfield Pet Hospitals...same product, different name). Frontline contains two chemicals: fipronil, which kills the adult fleas and s-methoprene, which acts as a growth regulator, helping to control other life stages of the flea. The concentration of fipronil is the same in both the canine and feline products, with the volume applied being different. The s-methoprene has a higher concentration in the cat version compared to the dog, but this isn't your primary method of killing the fleas.
Please don't split dosages between pets, or try to use the dog version on cats. Though the products are considered very safe, cats are extremely susceptible to toxicity from a number of flea products, and you may overdose your kitties. Splitting dosages is also not very accurate. If you use the products against label directions and then have a problem or find it not working, the manufacturer will not support you, help you out, or give you a refund. It is tempting to try and save money, but I strongly recommend against it.
I am also concerned about getting these products from eBay. There have been numerous cases of people purchasing veterinary products from online retailers that turned out to be from dubious sources. Many people selling these will get them from other countries, where labeling, shipping, handling, etc. are questionable. I have nothing against eBay specifically, and have been using the site myself to buy and sell for about seven or eight years. I also know that it has gone from being merely an auction site to hosting storefronts for reputible vendors. But you may not know what you're getting with an eBay transaction, and I personally wouldn't purchase a product like this from eBay. I have no problem with you getting things like this from sources other than a vet, but make sure you're using a well-known and trustworthy site. The benefit of purchasing these things from your local vet is you can ask them questions and get immediate help or support if there are problems. Always use caution when buying products from unknown sources, especially when your pet's health and well-being is at stake.