I love surplus stores. They're basically the tech version of a yard sale, with tons of stuff that would be nearly impossible to find or afford new. These are three of my favorites. Between them, you should be able to find just about anything you need, no matter how crazy the project.
Every aisle of the Los Angeles–area Apex Electronics is densely packed with boxes of resistors and capacitors, switches and knobs, and a multitude of other electronics basics. But what really makes a visit worthwhile are the hard-to-find items that might tempt you into something unique—things like synchronous motors, large capacitors and neon transformers. My latest find: an HP decibel meter from the '50s that I plan to repurpose in an upcoming data-visualization project.
Norton Sales Inc.
Norton Sales Inc., also in L.A., is ostensibly a rocketry surplus store, but it also sells loads of cheap pneumatic and hydraulic parts, which are good for anything from automating the clamps in your shop to building a computer-controlled fireball display. It is the source for high-pressure regulators, hoses, gauges and AN fittings, all at pennies on the retail dollar. My latest find: a bank of solenoid valves that I'll use in an upcoming fluid- power project--or with some propane flames.
From diesel engines to conveyor rollers, Surplus Center is the best online store I've found. The selection tends toward the mechanical, covering parts such as sprockets and chains, and it also carries a variety of electrical- and fluid-power parts. I haven't been there in person yet (it's in Lincoln, Nebraska), but it's on my short list. My latest find: battery-disconnect switches and charge meters that I used to make a solar-powered mobile office rig for PopSci.