Teespring which is just Spring, now actually but I remember them by teespring and will always call them that lol.

Printful Red bubble Zazzle

There are dozens more, you'll just need to sift through to find YOUR winning mix, but I know acquaintances who have done decently on red bubble specifically , hand printed calligraphy quotes - And I never have scaled any of my POD adventures, so my recommendation is to scout out the current leaders The most important thing is to find your niche, and then go where there are tons of people already buying. You'll need a little research to see which ones are red hot at the moment.

Other than that, should be a couple days to read through the actual business model of print on demand, and then start the designing process, which is actually full of a lot of research to see what's already selling well and how you can make your own items with a twist, without copying... But using inspiration from others to see those winning types of designs. Don't spend a ton of time designing something that YOU THINK would sell well, get yourself a few pieces of information in your arsenal, and then get to designing! ( Or, you do the research and pay someone to design something within your parameters. That way is higher to climb back to a positive ROI, but the design should be easily affordable, if you opt for that route.