You can actually make and sell shirt designs about coding, like this one I created teespring.com/electronics-dont-love-you-back or this one I also created teespring.com/270-percent the links are with big discounts. This site enables maximum 10$ profit per sold shirt which costs 20$. Of course then you need to somehow get people to see them to buy them, but if you have some websites I guess you will not have this problem.
So the sites are teespring.com for selling entire shirt designs and spreadshirt.co.uk for selling only the image for the shirt, at around 5$.