How do you decide what to print

I find a need for something. I need to attach my shop vac to this tool, so I’ll print an adapter. I need a bracket for this part on my ancient blinds. I’ll print a bracket. I need a case for a circuit board I just bought, I’ll print them.

What sites do you use to find free files?

At this point, Printables, though I don’t do that very often, I tend to model what I need in CAD.