I get mine from a company called Car Builder based in Kent.very good company with very fast dispatch and good on line shop, in fact they stock and supply so many things it is quite a wonderful catalogue, easy to spend too much ! ! web site: www.carbuildersolutions.com good luck
Depending on where you are, but I buy all my fixings, bolts etc stainless, and other metals from CBH fasteners in Redditch, Worcestershire. Check first if stainless is suitable for all your jobs. My understanding is that stainless bolts aren't as strong as some people believe.