I personally started with emails and cold calls, but unfortunately didn't get any results. You could send sample shirts to companies you think may be interested. That can get pricey, but the execs take a physical sample more serious than a design on their monitor, plus it can't get caught in their spam folder.
From personal experience, please be careful with solicitors who approach you. I once received an email supposedly from a large company that wanted to market my stuff. It turned out to be a scam. The guy who did it was an old roommate of mine, and was just out to hurt my company. I reported it to the FBI of course, but it would have saved some trouble to just check and double check anyone who approaches you first, and don't send them more than a sample without being paid.
You may also consider putting an ad on your website letting people know you're looking for a distributor, or maybe mention it in the Affiliates or Webmasters sections if you have them.
Theoretically the bigger your reputation on the Internet the easier it should be, so advertising and generally getting big online should help.
I hope this was of help. Feel free to send me an email if you'd like and maybe we could network more about it. I'm currently looking for the same thing.
-Armand
