I like the parts on
www.3dcontentcentral.com - look for the individual manufactures such as SMC. Also, we partnered with FIRST CAD Library to recreate a majority of Ed's parts in SolidWorks. If parts are available from manufactures in neutral file formats use STEP and import the geometry. File, Open, (select files of type STEP)
For a short cut, for parts that don't interact with any of your machined parts or moving parts, represent the part as a box with overall width, height, depth for its footprint. In the assembly, you can insert the actual weight in Tools, Mass Properties.
Marie