Ever since I made my dino shirt, I really have wanted to decorate a couple other plain shirts that I had sitting in my closet. I got it in my head that I wanted to do a polka dot shirt...so I did.
All you need to make your own is fabric paint (this is what I used) and a rubbery wine cork (like Barefoot wines). Just dip the cork into the paint and put it on the shirt. You might need to do a couple layers to get the look you want. I think I had to do about three coats. I also just eyeballed the placement but if you could always use masking tape to double check your spacing. Let each side dry for four or so hours...then it should be wearable...and washable!