There are many online resources and they are usally cheaper even if you factor in shipping. At least this is so for salt water. I belive pet smart has some good deals as far as equipment goes as long as you know what you are looking for, as you said. I even bought my QT tank at Wal - Mart LOL
. Sites I like are often refered to here on AA.
Dr. Fosters is a great resourse as it usally has detailed descriptions of products though they are not always the cheapest at least you can get a idea of what you want and how it works. http://www.drsfostersmith.com/product/shop.cfm?siteid=6
Marine Depot is another good one http://www.marinedepot.com/homepage.asp
One of members here on the GA forum runs Captive Corals online
Also reeferrocks is a great place for live rock HTH
I know some of these woulnt help you with fresh water but hopefully some one will chime in with some online fresh water site if not I will look some up for you!