Mark,
Whenever I replace nuts, bolts, screws, etc. I always attempt to use stainless steel. The exception to this would be something very "structural" that would require a stronger bolt; I can't think of anything off the top of my head but I'm certain there are some "grade 8" fasteners somewhere on the car (maybe just the engine) and in that case that's what I would use. Stainless is relatively weak and while great for small things, they don't bolt an engine together with it.
As for mere cosmetic or non-structural, stainless is what I have sourced and used. Places like all these sheet metal screws, trim screws, small machine screws, even larger things like those that hold on the bumpers. Trust me that either you or your mechanic at some point in the future will be happy you used stainless. I also use silver grade anti-seize.
One great source for small quantities of all kind of fasteners is your local nut and bolt store. Brafasco is a chain with locations around the central midwest and Ontario. There are others, I'm sure, near you. Search Yahoo yellow pages or similar, and you are bound to find an industrial fastener store near you.
Mail order, I have used the following:
http://www.boltdepot.com/metric-machine-screws.aspxhttp://www.fastener-express.com/and my all time favorite for cool stuff:
http://www.smallparts.com/One by one as I do any work, those zinc fasteners get replaced with stainless.
Thanks for the compliments on the engine bay. I didn't do it. I just maintain it...now where's that rag...
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America