If you have Wal Mart in your area try there. I purchased a bunch there for my trip this past December and only paid $5-$7 per shirt. They also had sevaral for my DS that were only $5.00 each.
I agree with walmart!! They usually have a great selection, and they change them every few months, so depending on how long you have to shop, you may get a few choices
If you have an H & M in the boys section I picked up a Mickey mouse shirt for 5.95, it is red and has Mickey mouse saying oh boy on the front, I think it goes up to size 8