I use the gel, because its said to work a little better then the spray and Winston has major tartar issues from his previous owner. Personally I like it because he's not fond and I can hide it (he loves his dog toothpaste so just stick the drop of the gel in his bowl & add a small blob of the toothpaste which I think must cover the smell up because he eats it up.)
I buy mine off amazon, I've bought from Entirely Pets & Litterboy Pet Products there. Litterboy Pet Products (might be in the marketplace section, the little link underneath that says ___ new/used from ___).
Litterboy Pet Products stocks it at 12.65 + 5.45 ship. = $15.10
I love Amazon for deals like that, since I get to shop at places that offer the product without a lot of overhead and I never have to worry about how credible sites are. None of the marketplace sellers get my payment info (just the payment to their amazon account) and if you ever run into a problem Amazon is quick to sort it out. I love Litterboy, when I had an issue with my shipping address they got back to me right away (and were able to help me out.) I got the product in the mail a LOT sooner then I expected too which is always nice.