There are various companies that will do stash for you. They'll all have a minimum order size, perhaps a sliding scale.
Colleges will have contacts you may be able to use, but there is nothing "official" about producing stash, just find a supplier of your choice, submit design and payment, recieve stash!
Can be very expensive for small runs though. Somewhere around £8 for a simple t-shirt isn't unheard of, and the nice warm hoodies are considerably pricier.