This is extremely niche, but I wanted to buy a t-shirt with this tweet:

I searched and found a few companies printing tweets on t-shirts, but none of them seemed to handle this particular case, where the tweet is a reply and not a standalone tweet.

have a look to Nostrasia ones https://awayuki.net/branch/products/t-shirts/

Do you sell them online or do you print and sell?

Get the paid for version of ChatGPT and have a field day talking to it in text and it generating images for. You can go back and forth and tweak to your heart's content. Couple that with even a basic image editor, like GIMP, and the world is your lobster.