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McMaster-Carr
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McMaster-Carr is fantastic!
I've been using McMaster-Carr for years. They always have the bolts and other hardware that no one else has. They also have very good pdfs with dimensions as well as CAD renderings. These are very important when your building prototypes and trying to figure out what may or may not work on a one off project. The best part is you get them either the next day or the following. Highly recommend!