I'm sure I've posted this info before but it bears repeating, for hardware, industrial type supplies and other mechanical odds and ends, you can't beat McMaster-Carr.

Their prices aren't always the lowest I'll grant you but their customer service is exceptional. I've been purchasing items from them for a few years now and have never had a single problem other than the one order in which I made a mistake that they rectified. I can order from them on a Monday afternoon and will usually have the items on my porch no later than Wednesday, usually it's next day.

There's a reason why people buy old McMaster-Carr catalogues on ebay, they're extensive, chock full of great info. Reminiscent in some ways of the Whole Earth catalogues in that they share lots of information as well as giving you the specifics on the items listed. They're also very hard to come by if you don't buy in volume, luckily their website is a online version of the printed catalogue and works very well.