If by medium you mean what does the artist use to draw, it
is usually a marker, or a set
of different tip-size markers. This is from my experience at AX.
If you mean what the artist draws upon, people typically give them a sketch board.
This is a semi-rigid piece of cardboard with a gold-trimmed, white drawing
surface and a sparkly golden surface on the other side. Drawing again from my AX
experience, these are sold at the convention merchandising booth, but you may find
them at some Japanese bookstores. They typically cost $1.50 or so.

I'll be going to Otakon, too.
---Trebuchet