Hi
Screwfix is a good cheap source for SS stuff, only in metric though, this is not a problem for most body related fastenings, just buy nuts as well.
Beware of using SS on anything that requires high tensile as they are not as strong as HT steel and tend to snap. They also have a curious property that the threads bind up easily when new. All this said you have no idea how much more pleasant it is slacken a nut and be able to spin it off by hand, I fit them wherever I can.
Re the corrosion thing, yes they will corrode ally slightly more than zinc plated ones ( higher up the galvanitic scale or something) but it's never given me a problem, rubber washer or a bit of silicone sealant judiciously applied helps.
Steve