The Japanese male sizes are smaller than American male sizes so that makes sense. There isn't one specific supplier of shirts I'm guessing because the shirts do tend to vary in size sometimes. I always get Japanese L and it ran a little small when I bought Haruna's T-shirt from Ultra Smart, but that L was HUGE when I got the "I <3 10th Gen" T-shirt, seriously I drown in it and I'm 5'7" and a little over 200 pounds. I have Ayumi's 16th Bday shirt on it's way to me, so we'll see how that fits.
Generally people stick to the Large size because a lot of times they don't sell Medium (weird) and small is really small. Large will fit most everyone. They do sell X-large though if some need to go up a size.
Just remember that Japanese shirts run small in general.