It doesn't take that much effort to make a custom t-shirts. I choose black because the white ones cause problems with colours.
1. Buy a cheap black t-shirt that fits.
2. In the stationery section of a department store like Wal-Mart there are packages of iron on t-shirt transfer sheets. Buy one for a black shirt.
3. Edit the image so that it will fit on a full page, filling up as much of the page as possible. Make a black border around it. Read the iron-on transfer instructions.
4. Print onto the iron-on transfer. Cut the corners so that they are rounded. Most iron-on transfers will say to cut out to the edges of the image, don't. You need the border.
5. Here's the difficult part. Ironing it too long will cause burning and wrinkling problems, too little and the image won't stick properly. One of those large flat machines to press pants works well if you have access to one.
6. Remember that over time and washing the image will get crinkly, cracked, and peel.
The first one's really falling apart, my wife keeps peeling at it whenever I wear it. But then I made that one for like the third TFCon I think.
Since then I had it "professionally" made, which ended up looking like shit and cost more than it did to make it on my own.
I will make another someday, along with a shirt containing every Transformers faction symbol.
Although I also have an interesting idea for a zippered hoodie and t-shirt combo decked out in the style of one of the pretender shells and inner robot. I might end up making that for the next TFCon I attend.