I want to make a shirt that i can design.

Loads of tailor shops around town - just go in and tell them what you want.

Good luck.

If you want just A SHIRT for yourself, then visit local tailers.

But if you want to mass produce, head over to Nimmanhaemin or Sirimangkalajarn Rd. There are many "Activities Shirts" shops around. They make lots of custom design tops for companies, schools and organizations.

Jipata is the largest of them. Don't know if there are any superior to others though.