tldr: it cheap to make you own prints, google it and go for it!
Imho:
Make your own graphic designs on your computer
Look for nice organic and/or fair produced Shirts (a small quantity should be enough to start with, dont you think?)
Than buy yourself a diy Kit für Printing Shirts
All of the order processing and fulfillment is handled automatically.
I do no marketing or advertisement beyond social media.
It's pretty nice. Sometimes I sell meme shirts that generate $3k in a month if I'm feeling particularly clever.
I was able to bootstrap this business for like $12 for a domain name and a free trial of Shopify which gave me enough runway to scale.
Been doing this for over a year, I still basically do nothing.
https://odomojuli.store/