I have a huge stock pile of blank t-shirts short and long sleeve in a bunch of colors, all primary. Sizes S-XXL.
..Also I have a heat press too. Print anything out 16 x 16" I can put on pretty much anything that's fabric. Only downside is that it faded after so many washes. My t-shirt took about 6 months to a year to fade.
I used a paper that comes up clear when pressed on and is nearly impossible to feel when wearing. A lot of heat transfer paper will have outer edges white if the whole space isn't used but the picture does not fade nearly as fast.