Looking for Links to merchants or orchestras that sell (or websites
that have for free -- better) Handel's Messiah in either (i) a
completely instrumental form, or (ii) a split track form where voices
can be eliminated by switching to the right or left track.

Thanks.

If you want vocals out, your best bet is midi files. You can download it here:
http://www.classicalarchives.com/handel.html

Thank you very much for your helpful comment. I tried the midi and
although I think that it will be great for learning the parts (e.g.,
isolating S,A,T,B), I'm looking for somthing that can serve as an
accompanyment to a choir.  The midi doesn't seem to do the trick,
unless the editor/player is limiting the quality of the sound I am
getting.

Chris