(Tips stolen from 4-ch /img/ for finding /retro/ images:)
* FTP servers. Find them with Mamont or NAPALM
* eMule. Go for *.zip files, since they preserve file modification dates. Good keywords include "HCG", "9801", "PC98", "画", Japanese titles of 90s anime.
* DC++. Again, look for *.zip files especially.
* Google/Bing/Yandex image searches. Add the "gif" keyword to bring up more old pixel art. I use wget to save files with their original dates.