Luckily, cyanotype is just about the easiest way to print a t-shirt, so I made my own!
I used the powdered cyanotype chemicals - looking at Third Man's video, I think they used the liquid version, which is probably easier (less weighing out cyanide in the kitchen involved). The liquid is part number 07-0091 from same dealers.
I used it exactly as in the instructions (PDF), but with both dichromate contrast enhancer and a hydrogen peroxide oxidizer/darkener bath. (I probably only needed the latter.) It only took a four minute exposure in full sun here in So Cal in early October, but probably much longer in winter in most places (so do a test on an old sheet before using an expensive blank t shirt).
My test sheet was a drawing I had to do in my sketching class. Yes, it's a copy, but it's my drawing if you know what I mean.
For the negative, I used the picture on the Best Buy CD box, enlarged, contrast enhanced and with the "W" removed in photoshop. I reversed the image and laser printed it on an overhead transparency sheet. (Special stock these days - Apollo CG 7060.)
I'll order a bunch more t shirts and do some other negatives. (Before anyone asks, Third Man would no doubt frown on me making a lot of copies of their limited edition item, so I won't be doing any more.)
A screenshot from Third Man's video showing their process - the negative sheet is being pulled away from the exposed cloth (it's not blue at this point in the process):
A photo (ganked from Facebook) of an original, genuine t shirt from Third Man. (The image is slightly larger and much less saturated than mine.)