Note: If they are not in .stl format, you can always convert them easily online here : http://www.3dtransform.com And if the .stl doesn't slice correctly in your slicer, check here : Fixing .stl files

Community based librairies : http://www.thingiverse.com/ https://www.shapeways.com https://grabcad.com/

Manufacturer based librairies :

http://www.igus-cad.com https://www.elesa.com https://www.mcmaster.com/#machine-component-knobs/=16wyoq3 http://www.te.com/usa-en/plp/knobs/Y30e7.html http://dimcogray.com/product-options/push-pull-knobs-and-handles