East Coast US based supplier specializing in fasteners and only fasteners.
Notably their navigation is terrible and their search is for part numbers only and not item searches.
You can save and retrieve carts for separate BOMs (though they do not currently have an easy way to share BOMs with other users).
You can add multiple items to the cart from the same "category" by using the not so obvious "Add All Entered" button in the upper right of the category page.
For RepRap purposes you'll probably want to skip past the first left navigation bar and get straight to the metric machine screws area.