Which brand/style t-shirts are you interested in?
I think you can get a wholesale account at AlphaShirt.com where they sell Hanes Tagless tees. Although the "tag" is screen printed in the collar, there is no hanging tag.
You can also have just about any t-shirt brand relabeled with your own label through your local screen printer or seamstress.
Companies like Continental Clothing and AlternativeApparel will sell tagless tees I think.