1. Nuts, bolts, pretty much everything: McMaster, Grainger, MSCDirect 2. Tooling & fixture components: Carr-Lane 3. Machine tools: Iscar, kennametal, see #1 4. Automation stuff: Misumi, see #1 5. Metal bar stock: onlinemetals.com, see #1 That's all I can think of for the moment, but there are a lot more that I go to for specific stuff.