I think one good place to go online (or in person
if you're close to Sherman Oaks, CA) is Van Eton
Galleries.  They're at http://vegalleries.com and
Anastasia is in the Don Bluth section.  All Van 
Eton has right now is a drawing of the three main 
characters, but you may want to give  them a call, 
they often have stuff in but not on the site.
Another place to check is the American Royal
Arts Corporation at http://www.ara-animation.com
I took a look and they have lots of Anastasia, but
I only saw one drawing of Bartok.  Those should
get you started! :)

Jen