From the Super Mario Wiki
This is a gallery of Mario-themed clothing.
Gallery of shirts
Many of the earlier Mario T-Shirts were given away as pre-order bonuses. These times include:
More recently, a combination of 80s nostalgia 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
Hats and shoes