McMaster Carr has certain items available in CAD, either 2D or 3D (I haven't tried the 3D). This tends to be stuff like bolts and pipe fittings. There's not a single repository of all of it, but handy for specific items.
Check with manufacturers of the items you're using. I've found gate valves and Bilco hatches, for example.
Pet peeve: Finding piping SYMBOL libraries instead of piping drawings. Anyone can stick an X on a line and call it a gate valve, but actually drawing a gate valve is more involved.