We bought a white t-shirt each from H&M (£2.99 & £3.99) to try out the Ikea Loppstarr Textile Pens. They can be found in the textile department for £1.75 a pack.
Son asked me what sort of t-shirt he should make. I said, "what would you really love on a t-shirt that you can't buy in the shops?" So, of course, he drew a giant ham & cheese Subway, a stage, and a disco.
I picked a t-shirt which was more stretchy than his plain, white cotton one, which proved to be a mistake. Plain tees are easy to draw on, but stretch fabric is really tricky. My t-shirt didn't come out too well, I couldn't draw lines just make dots to put together a picture. So my advice is go for very basic cotton tees, or fabric. I gave mine a Ridgway Homemaker plate design.