I'd imagine that
www.thatpetplace.com would have some of the best deals online. Never shopped there, but I have bought stuff that their main website,
www.thatfishplace.com, and they have (well, had... haven't had fish for 2 years, so I dunno if they changed) the best prices online for fish stuff. I'd addume that their general pet store has similar killer prices (usually 1/3-1/2 off of full retail prices).