Oddly enough I've actually spent the last few weeks looking for almost exactly the same thing.
I'll be following this thread if anyone else has ideas.
EDIT: Forgot this, but this claims to be a Nitrate-sensitive probe:
http://www.vernier.com/products/sens...rodes/no3-bta/
Vernier also makes a NH4-sensitive probe but if you want to estimate NH3 you will have to acidify the sample.