I use a local company called Nashville Wraps. They are very big and ship all across the country.
I do lots of face-to-face selling (since I haven't finished my site yet!) and wanted something to put shirts in instead of just handing customers a shirt and sending them on their way. So I got some plastic merchandise bags with handles, had my company name and logo printed on it, and put the shirts in the bags. I also stick a small brochure with my other products in it and my contact info.
Their prices are pretty good and they offer custom printing. You may be able to get a better deal on the printing from a printer in your area, though, but the prices on the bags are very competitive.
Check them out: www.nashvillewraps.com
You have no idea how much this post helped me...thanks!!!