Try contacting Jeff Melzak at Case Western Reserve University
(CWRU).  He can be reached at 216-368-3628 or jeff@mems5.cwru.edu;
he is the Microfabrication Lab (MFL) manager for the MEMS effort.
CWRU did a lot of nice polishing when I was there; but I am not
sure if they offer that as a service.  Also, you could try PCA
(Polishing Corp. of America) at 1-800-672-0060.  I have not
used them, but I know that they do contract polishing (including
nitride).

Good luck.

-Kevin