Are you looking for a specific design on a ringer t-shirt?
Lots of t-shirt sites sell their designs on printed ringer shirts (check through the many cafepress stores or spreadshirt stores)
As for blank ringer t-shirts, a lot of manufacturers carry them including Anvil, Alternative Apparel, AmericanApparel, Hanes, etc.
To get ringers printed, you could go to just about any printer.