...and all other kinds of garments since 1976, including costume graphics for a bunch of movies/tv shows/music videos and live stage performances.
I started as an airbrush tee artist, and developed various hand-painting techniques using hand-carved - and now laser etched - foam printing implements, plus I use silkscreen printing methods, including spot colors/CMYK 4-Color Process.
But the easiest, and most likely affordable way to transfer an original painting such as yours to a tee shirt or other garment would be to find a Direct To Garment printer.
With a high resolution (300dpi) good quality digital photo, or better yet, a 300 dpi scan of your art, you simply size the image to the dimensions you want it to be on the shirt/garment, and save it as a .png (Portable Network Graphics).
The DTG printer uses the .png file to digitally print your image directly on the garment, much like an ink jet printer prints to paper. The ink is very durable, but you will see fading after a certain number of washings.
If you don't have Photoshop or some other graphics program to convert your digital photos/scans to .png format, I think most DTG printers would be able to convert your (most likely .jpg) to .png, probably for an extra charge, hopefully no more than 10 bucks. The cost to print one shirt should be in the $15-25 range, plus the cost of the shirt, maybe another $15-25. These are only estimates, and I could be off, but maybe for one shirt you're out $30-60.
For comparison, if you went to a competent silkscreener, this type of painting would most likely be done using CMYK (Cyan, Magenta, Yellow, Black) 4-color process, where those colors are screened on top of each other, and blend together to look like a full-color image. This would cost upwards of $300-400, maybe more, to do color separations, print films, burn screens, formulate the ink and do the first strike-off. No screen printer wants to do all of those preparations and work to print just one piece, so they often require minimum quantities before they'll take the job.
Lastly, printing - by any method - art such as a watercolor painting onto a tee shirt works best on a white ground color. Dark ground colors are trickier to do, and the vibrance of the art is diminished.
= new reply since forum marked as read