Just wanted to let everyone know, the Elf Mates shirts you can find at Dollar General fit Barbies very nicely. I added some snaps to the back of mine. They make a really cute addition to the holiday wardrobe. There are three different ones, blue, purple, and green, all with different designs, and they're$3 each. I haven't tried them on anything but a Barbie yet, I will check back in with my results.
Edit: They fit Integrity Toys girls nicely, though they are very loose at the waist, they fit an Obitsu 27cm girl, though they're very baggy (if you're going for the "just stole my boyfriend's t-shirt" look, it's perfect), and they make a perfectly adorable oversized t-shirt to sleep in for an Obitsu 21 cm girl, especially if you guys it down off one shoulder.