I've used these sites in the past.

Care A Lot Pets

Drs. Foster Smith

Be aware that Australia has a Frontline Plus for Felines that is NOT approved to be used in the US and it is a lot less expensive. I found this out by doing a search to find the least expensive way to treat my 8 as well as my mom's 9 cats.....I know that these two sites listed above sell the US version of the 3 and 6 packs.

I have also had experience using the BioSpot and I would NOT recommend this to anyone. Stick with the Frontline Plus or just the plain Frontline is you are not concerned as much about ticks.