While those are very nice things to get signed, 
it's not nessesary to get that sketched on.  In 
fact the artists will draw on whatever you give 
them.  Alot of fans will have a sketchbook and 
will have the artist draw on one page of the 
sketchbook with whatever drawing impliment the 
artist prefers (pencil, pen whatever).  Then the 
fan has a whole sketchbook he can flip through of 
sketches by various artists he has met.  
Personally I just get decent sketchbook paper 
(with an ok tooth and thickness) and if the artist 
does not already have something they prefer to 
draw with (they often do) I offer them one of my 
pencils.  I keep my sketches separate in my 
celbook, just like I would any other animation 
art...  although keeping an intact sketchbook 
seems like a lovely idea to me too.