Where can I get renders/models of common parts like screws, bolts, etc. As well as electrical parts like terminal blocks for example?

Generally speaking you can get them from the component suppliers. Check the website of the company you'd be buying it from or their competition's website. In the case of what you were specificily asking for McMaster-Carr would be a good place to start.

If you can't find the model for what you're looking for on the companys website another good place to check is 3D Content Central, they have a ridiculous number of companys who have CAD data for their products available on there. You can sometimes even find CAD data for more unusual things on there made by various users also.