Mario Shirt

Mario Shirts are usually T-Shirts. Ocassionally, you'll find a sweat shirt.

Pre-Order Bonuses
Many of the earlier Mario T-Shirts were given way as pre-order bonuses. These time include
 * 1996 Nintendo 64 Pre-Order Bonus (Later available through Nintendo Power)

Recent Merchandise
More recently, a combination of 80s nostaligia and geek chic made it profitable to sell Mario shirts to the general public. Web sites like 80's Tees and Cafe Press as well as stores such as Hot Topic sell this kind of shirt.

Some people have slightly changed Mario's image in order to get across a message (and possibly around copyright laws). For example, this t-shirt conveys a Christian message.