Both great choices. Also Northern Tool, Rutland Supply, and Aircraft Spruce. Your local hardware or home builders supply don't carry a big selection of parts for someone who is engineering homemade drive systems. You have to find an industrial supply or bearing house to find the varied sizes of belts, pulleys, chains, sprockets, bearings, etc. The most obvious place to find these suppliers is in the manufacturing or industrial part of any city. It's nice to be able to see what you're buying, otherwise mail order catalogs are the best sources.