This is probably not the cheapest or easiest way to do it, but this is how we did it for HHDRC t-shirts.
There is a shop near the big photo shop on Petchkasem Rd (the one next to the night market street) that sells t-shirts at the front part of their shop. They are also near the newspaper shop. They sell plain white t-shirts in farang sizes (no good for kids or small Thai ladies). We bought a load of t-shirts there & took them to a shop near Putahracsa (little shop that sells sewing supplies, they have a cloth banner out front in English saying "We do embroidery"). They will make up a "box" with whatever you want on it (I think the box costs 500 bt), then it's a cost per t-shirt embroidered. We had 3 boxes made. They keep the boxes forever, so once they're made, it's just the per t-shirt cost. Nice quality of embroidery.
As I said, not the easiest or cheapest way, but it's a possibility if no-one comes up with anything better.
"The question is not, can they reason? Nor, can they talk? But, can they suffer?" - Jeremy Bentham, philosopher, 1748-1832