If you're in physical proximity to a store that sells used instruments or odd electronic junk, then give it a visit. The San Francisco bay area is the mecca for electronics junks stores, but they are also scattered in odd places around the country. I look in the yellow pages when I'm in a new city.
Stores and online instrument dealers overlap a great deal, so check out the web sites of online dealers. Oftentimes old instruments or low-value parts and accessories aren't listed the online site, but can be found by rummaging around in the store. Also look under “surplus” in my links page.