We have had a shirt system set up for several years, actually. We first made them when we had our 1st Brushless bash(about 5 years ago). We just made the design and sold them through a 3rd party - the shirts aren't what I would call inexpensive, but they are decent shirts(I have some that are several years old that I wear daily for working and/or racing - I am actually wearing one right now!). I wear the Polo shirts when I do RCX and IHobby shows. :)
I have looked into actually carrying T-shirts in house - I could probably do much better on pricing with some volume, but I haven't really "gone after it". :)
http://www.cafepress.com/rc-monster