Aquabid is one place to look. I have had great success with craigslist when it comes to used aquarium stuff. Try using "fish" or "aquarium" as your search in the for sale section. There is always a lot of variety in my area and it changes frequently.
As far as "deals websites", aquarium products are a niche market, which means low volume demand, which means expensive. Because retailers don't get a price break unless they buy in VERY large quanties there is no incentive to offer deals or sales.
Amazon is one of the few places where aquarium products can be sourced consistently below retail cost. I would highly recommend checking them out next time you need fresh supplies.