Everything-in-the-world parts supplier McMaster-Carr is awesome…if you know the name of the part you're looking for. Years ago I had a small, racetrack-shaped piece of flat metal symmetrically pierced with two threaded holes, and I needed more of them. It took a lot of oddly-worded Google queries to figure out that it was called a "weld nut," and then I could finally figure out where to buy it.
Amazon has rolled out a new feature on their mobile app that aims to solve problems like these. Called Part Finder, it has you photograph a fastener with your phone (next to a penny for scale), then figures out what it is, how big it is, and offers up some buying and sizing options:
Thus far they can identify roughly 100 different types of fasteners.
What I'd love to see next: The system able to tell the difference between 10-24 and 10-32 threading, or a 5/8 hex head versus a 16mm.