Where to buy or salvage Gears/Cogs?

I need any size of gears above 0.5 inches in diameter, but I can't seem to find places that sell them or things that have gears to salvage. Can you give the links to the items with the cheapest costs?
Update: Max size around 8 inches in diameter. I'm looking for worm gears, bevel gears, regular wheel shaped gears, turning gears (reducing gears), anything, preferably metal. I'm building a small toy mechanical system about 2 ft cubed, and I don't want to spend more than $20 on gears