I like to make my own with fabric paint.
It is a bit of work, but i find it quite enjoyable. First you want to draw your design into something that could be made into a stencil. Make sure you dont have any islands (floating pieces that would fall off when you cut the stencil)
After you have your design, transfer it to some thick stencil “paper” and cut it out. You can use a razor but it will start to hurt your hand a bit if you have a real intricate design or a lot of cutting. For this reason i like to use a soldering iron knife it gets nice and hot and cuts through the stencil like butter :).
After you have your stencil cut out your ready to put it on the shirt. To secure the stencil to the shirt and prevent bleeding, spray the back side of the stencil wtih some spray adhesive (a little bit goes a long way, if you use to much your going to have gummy residue on the shirt for quite some time.)
Firmly press the stencil down onto the shirt, and make sure all the inside edges (area your painting) are stuck to the shirt. Now just paint inside the stenciled area. I dont know how every brand is, but ive found with my fabric paint you really have to push the paint into the shirt with the brush.(which reminds me, make sure you have a thick piece of cardboard or something on the inside of the shirt.) Let it dry for a bit then pull of the stencil. (if you wait too long, your going to have a hard time getting it off due to the glue)
So yea thats all it takes. I know it looks like a lot, but its really not, and once the stencil is made, you’ll have it forever for whatever you want. So replicating more shirts for say, profit, wouldnt be hard.
Heres two of the shirts i made