1. Metalliferous www.metalliferous.com sells a good assortment of metals in sizes and shapes common for hobbyists.
2. All Metal Supermarkets www.metalsupermarkets.com you can get small pieces and quantities of just about any metal cut to the size you need, while you learn from their helpful material descriptions.
3. OnlineMetals.com is another great resource, and you can get a 5% discount if you access the store through the Maker Shed site www.makershed.com/v/metals/
4. Speedy Metals www.speedymetals.com has a good selection and good prices, but searching the site is less beginner-friendly than on the preceding sites.
Don't discount sets like Erector Set and VEX Robotics Design System for structural metal parts that are easy to configure.
5. 80/20 Inc. www.8020.net and Item www.itemamerica.com sell grown-up versions of these configurable structural systems, and the MakerBeam www.makerbeam.com is an emerging open hardware version of the same.