Some places require an open account, and with a good D&B number, and a couple of references, that can be done. Some don't care, and are set up for a "will call" pick up by a cash/check customer. If you have a tax number for resale, then you should furnish it if the metal is going into manufactured goods. For shop projects, tooling, workbenches and the like, the law is a little vague and some of that can be tax exempt under "use". If in doubt, pay the sales tax or consult your CPA.
I don't think there is a single source supplier for everything a machine shop normally buys. If you need stainless, go to a stainless specialty house, carbon/alloy, go to those guys. Brass/bronze/copper, etc, go to them. There are a couple of the larger companies who sort of bridge the gaps, but you generally are better off with stock availability and price going to the people who specialize.