At around the mid point in the OVA series I 
believe that a different group took over. They had 
different companies do the animation for the 
various remaining OVAs. Actually 10-11 are the 
worst in the whole group. 12 and 13 are actually 
very good and some are the cels are quite nice. My 
commentary on the series can be found here if 
people who have not seen the series are interested 
in knowing more about it:

http://www.dpo.uab.edu/~marcus/cels/Hakkenden/p-Ha
kkenden-comment.html

As to where you can get cels from the show, they 
are around, but usually not displayed since cels 
from the show are not that highly sought after.

Catch you later,
Marcus