If you have one of the shirts you like, you can look up the supplier by entering the RN number into the FTC RN# Search database. A link to it is in the far left column of this page under 'Resources'
It is possible they are blanks, but more probable they make their own, or have their shirts made for them. In which case, the RN search will not help you much. But it's worth a try.
Regarding your second post, if you don't want garments that will shrink, look for 100% polyester garments. Here are posts discussing the cheapest places to buy
Wholesale T-Shirts and Other Imprintable Products - T-Shirt Forums