I think in January of this year I started making t-shirt designs for sites like redbubble (don't try to search them, they are too ugly to be seen hahaha, I was just starting how to use illustrator that time). The one I like most is Threadless, but mostly because there are design contests where I can share my art and gather some following, and you can also create your own shop there, if they like your design, they will put it in their official shop + $1k + commission (it doesn't have to be the most rated art, tho). Oh and they featured the designs they like on their social media as well
The only bad part is that it seems no one buy those prints or maybe my art is too poorly made to someone actually buy it
, I haven't updated that site since I started my comic tho, but I'll try it out later
On Teepublic I think I sold 3 t-shirts so far, without any promotion, they also have a facebook group where you can promote your stuff, and it seems that fanart (especially marvel) sells more, but you have to deal with DMCA