Here’s a tip that I wish I had thought of before I dumped about 100 dollars into aquarium gravel and rocks at the local pet store. Unless you’re looking for bright pink aquarium gravel or something, chances are you have an even better source than the pet shop: your local home improvement store.
Places like Home Depot carry gravel of all sizes and colors in their landscaping section. Some are even available on their website. And it’s a heck of a lot cheaper, you could end up paying a fraction of the cost for the same stuff because it’s not in a smaller bag marked as aquarium substrate.
Landscaping stones and lava rock also make for excellent and inexpensive aquarium decorations.
Of course that only applies to regular gravel, rocks and sand. If you need something more specialized like planted tank substrate, you’re better off sticking to aquarium stores (and watching this site for deals on the stuff.)