Okay so more or less like prototypes? If you have the funds I would go with screen printing. Apparently there's some other method of printing that RedBubble uses that @Anime was telling me about, you could ask him about that one if he has any information on it cause I do not. I hope it all works for you. I had a clothing line for a while and then I just let it drop off. I'm currently on the design team for another company now though and for tees we use screen printing.
RedBubble used DTG printing which is short for "Direct to Garment" printing. As the name suggest, they print the design directly to the shirt. This allows for companies like RedBubble to do custom orders fast and efficiently and with no minimum orders.
The quality is the worst out of all the printing techniques and RedBubble just fucks it up even more. Plus their prices are fucking insane... $28 a shirt? No front and back customizability? Yeah no thanks.
I'm currently looking at CustomInk. Their printing process should be along the lines of DTG but they seem to have a tighter quality control team that actually has eyes. Their prices are cheaper then RedBubble too and you can print on both sides. They also let you pick the shirt material as well...