From what I've heard the Frontline is the best product. We went to buy it from a farm & animal feed supplier as it was much cheaper than the vets, but it turned out not to be the frontline combo one...so not much good for keeping all the bugs off!
I'm suprised your vet has stopped selling it, but then I've not heard of Advocate, so I guess it's likely to be as good.
I wouldn't bother with the cheaper stuff though, I've never heard of it working effectively.

There are 'alternative' ways that a lot of people swear by, if you fancy trying a different route.
Here's a website I use for my shampoos & flower essences.. Holistic Pet Supplies - Complementary Remedies for Dogs, Cats & Horses
Very good company!