If I find that I need anything, I try to see if EBAY has it first. EBAY is like one big online nationwide garage sale. You can get stuff dirt cheap. However, I will check other websites and store ads as well before I decide to buy something. By shopping around, you can usually find what you're looking for at a much better price.
BONUS: EBAY is also a good place to sell your unwanted stuff for extra cash.