Tell people exactly what you think - make your own T-shirt. Creating your own apparel has never been easier. Here are some options:
The Basic - If the budget is low or the deadline is soon, this is your best option. All you need is a t-shirt (white) and a sharpie (black)
The Puffy Basic - Same as above, but with colorful "puffy paint." It creates a nice 3D look.
Classic Iron-On Letters (see photo) - A little more expensive, but legible (if you can spell correctly).
Digital Design - Make it on a computer and print it out on an iron-on transfer (just remember to print out a mirror image).