I bought it online, you have to compare prices + shipping, I don't recall where I ended up buying it. It runs $40-$50 for 500 mls.
It is 1-2 sprays per lb and you shouldn't reapply more than every month. We don't have a bad problem here at all so I can't really vouch for the efficacy, but it does what we need it to do.
You do need to spray the entire animal, including places where fleas like, like belly, and make sure they get wet so be prepared to get rubber gloves and do it outside or the dog will get everything wet until he dries, and it somehow is horribly toxic to humans even though it's prefectly OK for dogs so be careful when applying.