What do you do when missing screws?

I have been meaning to post a topic on this. What do you guys do when you're are missing screws for parts you need to install? I need to install the following:

1. 71 duster tail lights
2. Lower grill panel
3. 69 Valiant grill
4. Headlight Bezels
but basically you get the idea. Some of these parts I never had screws for, so I wanted to know what's your method when getting screws for these parts and finding the correct ones for each part?