I got a couple long sleeved ones from Walmart. I think they were $5 or $10 each. I find they are too big for my crew and haven't found any smaller ones so when we paint or do water inside, they do it in their diapers or shirtless. Water outside is done in bathing suits.

I keep meaning to go to a second hand store and buy long sleeved shirts and just cut them down the back and attach a piece of Velcro. They will get stained but it won't matter and I can just wash them in the washer.