I want to make some early 90s Indycar skins... where can I find some good pictures?

Here you go drinklime.


Here are some Champ Car photos.
http://www.champcarworldseries.com/Event/EventPhotos.asp

Here are some IRL photos.
http://www.indycar.com/multimedia/photos/

Hope this is what you was looking for.

I use this site a lot for replicating liveries from just about any major real racing series on the planet. http://www.motorsport.com/photos/

It has a great search function where you can find photo's by series, events, driver names or track.
It also has photos from previous years of racing so is also good for resurrecting older liveries.