>1) Did Nagi just appear in Tenchi Universe?  (I 
>haven't seen any of the pretty Sammy stuff...)

yes, please someone correct me if i'm wrong, haven't
seen all of the Pretty Sammy yet.

>2) Why is it so hard to find a good Nagi cel?  
>(Anyone have one they want to sell?)

Nagi's aren't that hard to find and what's more
surprising for me, every time i see them, is that
they aren't expensive - for example one seller i
remember:
http://www.rosearts.com/rose/christmas10.html
have you tried the dealers apge at cels.org ?