Custom Sweatpants.?

That's a problem with screen printing, it's really only economical to do several items. You could contact the art department at a local high school, trade school, or college. They even learn to do this in the Technical Education course in our middle school. If you only want letters (not a custom design) you can get that done at a local sporting-goods store; they put names etc. on back of jerseys. T-shirt shops do this work too.