I'm not sure about an erasable sharpie (you could probably find it on google, or on the internet), but erasable markers, such as the Crayola Erasable Markers, can be found at Amazon.com, Office Max, Staples, or your local stationary store.
On a side note, rubbing alcohol, not the drinking kind, and Mr. Clean's Magic Eraser remove sharpie marks, as long as it has been written within a few hours.
Cheers,
JTM