Check EBAY, I've gotten several for my collection for reasonable prices. Some good finds to be had. Mr. Hamish is correct, Cotton Traders are top notch. Barbarian makes a good shirt as well.

Also check these stores. I've ordered from all of them and they're all good folks to deal with.

www.ruggers.com (my team sponser) ;)
www.ballsout.com
www.rugbystore.com
www.rugbyimports.com