I would like to download a free symbol library of common bolts, nuts, hex cap screws, allen cap screws etc. Does anyone have a reference to one?
2D or 3D? Do a web search. You'll find many examples, for "free" or, for a fee.
I would suggest, as it has been my own experience, that you might be better creating your own selection, as suggested by Roland.
It's a good way to learn, and, is very quick, once you've got the hang of it.