While I don't get asked to draw many Turtle sketches at cons anymore, I do usually have a few pre-drawn ones with me for the devoted TMNT fans to buy.
I did these for Megacon. Just for fun, I drew them on the backs of heavy cardboard that were originally the tops of Turtle sneaker boxes we had in the studio several years back. So they are kind of like giant trading cards with an original drawing on the front and turtle graphics printed on the back.
The graphics on some are even in French, which I think is extra nutty.