One of the best directories for clothing manufacturers is the Apparel Search directory. In the clothing factory section, their is an area to help you find t-shirt factories.
In addition to the above mentioned section, you may want to check the shirt manufacturer section as well. Some of the shirt factories produce t-shirts but may not have been specific enough when they submitted their company details to the directory. If that is the case, they may have been listed in the "shirt" section only and not in the "t-shirt" section. You may find additional resources in the following area.
Do you own or work for a T-shirt Factory?