I think Jasonda summed it up pretty well.
You just have to find a quality blank t-shirt that you like. The brands you mentioned above probably have custom blanks made by the 1000's to their specifications that probably aren't available for general wholesale.
Article1.net, continental clothing, alternative apparel have some nice blank t-shirts that you may want to purchase samples from.
Then you have to find a water based ink screen printer to work with and you should be good to go
