I don't know the right name, but the screw cap on each tire, I lost one. Where do you buy those things?

 

Autoparts store. Cheap cheap at Autozone. Unless you go for the uber-Kewl led ones.[/sarcasm]

Or steal it from another car.
j/k!

 

valve stem caps are what they're called.

 

You can get a set of chrome ones at AutoZone for a couple of bucks or just plain Jane ones for even less.

You could also find an old bicycle around the house and take one from there too or even from your spare tire in the trunk.

 

You don't really need them either, my car and my bike are both missing a few. No biggie, air doesn't escape or anything...

 

They aren't really there to keep air from leaking out, they're there to keep the valve stem free of dirt and debris so that when you do need to check and/or add air, the valve won't be stuck or gummed up.