Do a search for wholesale t-shirt supplier. Don't use any who don't ask for a resell license, you probably aren't going to get the best prices.
I use:
www.alphashirt.com, they have a great selection of products, good prices, and wharehouses all over the middle and eastern parts of the country.
Another I use is:
One Stop - America's Best Supplier, they seem to specialize in more of the athletic stuff. While they of course carry Gildan, Hanes, and Jerzees, they also stock Bella & Badger as well.