In an earlier post, I advocated for patronizing your local stores when purchasing aquarium products. However, sometimes ordering online is a better option. To that end, I thought I would list a few of the sites that I've ordered from or sites that hobbyists I know support. These are general supply sites, which means they carry a wide-range of aquarium products. There are certainly other sites, including many that are quite component specific (e.g., tanks, tank lighting, filters), so this isn't a comprehensive list.
Amazon - Surprisingly, Amazon's inventory is pretty rich in aquarium products.
Aquarium Connection - As the name suggests, this site is specific to aquarium products, though it also sells pond products. Very comprehensive, including articles and product reviews.
Big Al's - A general pet site, but I have used it many times. Have always received great service from them.
Drs. Foster and Smith- A general pet site like Big Al's with a comprehensive aquarium inventory.
Jehmco - Not as large a site as the others above, but sells quality stuff.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub