Eric wears this shirt in a whole bunch of pictures we have of him including his senior photos. He also wore it over his Natural Selection shirt on the morning of 4.20.99. A similar shirt can be found HERE. 

The only time we see Eric wearing this specific shirt is in the yearbook photo where he is sitting in the cafeteria. It is probably made of heavier cotton similarly to #1. A similar shirt can be found HERE. 

A shirt for the 1997 KMFDM tour schedule for the release of their album Symbols which included one of Eric's favorite songs, Stray Bullet. He wore this shirt during the "Eric Inside Columbine" video. They are tough to find nowadays, mostly because of their association with Eric, but they can occasionally be found on eBay.   

Eric wears this t-shirt in the "Radioactive Clothing" video. As far as I know we do not see him wear it again. Sometimes they can be found on eBay.