Alot of music festivals sell booths where you can sell your stuff. I think that you would be better off selling the shirts out of a booth than handing out a flyer for a website.
People will likely throw the flyers away, or loose them. I dont think you would convert that many sales. In direct mail, where you send mail to a targeted demographic, the average is 1% responce. In an untargeted advertising technique like that I think out of 1000, you may pick up a few sales if youre lucky. Like Rodney said though, if the demographic at the festival matches your demographic, it might be worth the while.
If you need flyers, I bought some from
www.artandadvertising.com/flyerprice.html . 2500 for $180 was a good price, and the flyers looked good too.