in Japan, the shikishi board is the typical 
autograph / drawing medium for signatures and 
sketches.  Personally, I like to get my own acid-
free, heavy-duty art paper instead because the 
shikishi board tend to fade into yellow after 
certain years as well.  I guess it's depending 
the humidity and temperature in your area too.

Whether you can get sketches from the artists are 
really up to them.  I was in a few small cons 
before, some artists just didn't feel like to 
draw even the lines are short.  Different 
reasons, some artists draw very slowly, and some 
are tired after the long flights, and some just 
prefer autographs only...etc.  

As you may know, in Japan, most popular artists 
usually give out only autographs for the Japanese 
fans, they rarely draw sketches.  They actually 
tend to draw more when they meet the foreign fans 
in US or Europe, that's why some Japanese fans 
are jealous of us, hehe.  On the other hand, if 
you go to Comicket in Japan, almost every 
doujinshi artists will draw you nice sketches, 
but then, they're not famous yet :)