When it comes to t-shirts and printing, don't fool around. If you want anything done seriously in Japan, go strait to Sweatshop Union, do not pass go.
As the name indicates the print man himself, Matthias, works hard to give you good quality results in decent time. Sweatshop Union is a little different from other print shops in Japan because they provide plastisol prints; designs that don't fade in the wash, don't peel and even remember their shape if they've been stretched. The shop also has a number of specialised inks, including glitter inks (the most popular being silver and gold), a glow in the dark ink, and even a whiteboard ink, allowing you to draw on your own t-shirt. You can order any colours you want in the CMYK range (even special colours), and print up to 6 individual colours in the same design, really expanding the possibilities.
Matthias' wife, Miyuki takes orders and deals with inquiries (both in English and in Japanese), and is not only friendly, but helpful and patient with customers. One of their sons, Mitsuwa, is also a regular at the shop after school hours, and that reallty gives the place a homey feel. You know when you walk in there that it's not just another typical company; it's family, and you can't go wrong with a family business.
Screenprinting can be an expensive process for a small run (like, 1 or 2 shirts), so you might want to consider your reasons for printing before you rush in and order. More information about the process and the whole screen printing deal can be found on their website (here).
Sweatshop Union also colaborates with the artist PKMFA on an original label It's Our Thing, producing it's very own attractive designs. I'm a proud owner of two items from It's Our Thing, and they're so good, my wife dosen't like it when I wear them around the house.
I was lucky enough to get some great working experience at Sweatshop Union, and I learnt a lot about the silk screening process while I was there. This is exactly why I can say, with confidence, that Matt produced the best quality screenprints in Kansai, if not Japan.
Best shirts, right here in Kansai.