Colored sketches are hard to get any way you look 
at it.  If you are really lucky color ones will 
be for sale from time to time, very rare though.  
Some of the color sketches, and the better black 
and white sketches I've gotten in the past were 
from trading with the artist other sketches I 
had, cels, etc.  

Normally the black and white sketches can be done 
quickly, though more detailed ones (like the one 
Watsuki did of Kaoru, or the Milly (Trigun) by 
Yasshiro) will take longer.  A friend of mine got 
a really nice Jubei Chan sketch from Yoshimatsu 
over the summer (color and all) and said it only 
took about 10 minutes from start to finish to 
draw it for her.  The Pirotess one I got from 
what I was told probably took a few hours.

Now getting a sketch at a convention is 
exponentially harder than it used to be.  When I 
started collecting you would go up to the artist, 
ask for a sketch and normally they'd be overjoyed 
that you recognized them.  Now you have to deal 
with all the lottery BS :P  Though there are 
still shows out there that practice the old 
school method of waiting it line!