I hate to say this but there was a cel of him on
Yahoo Japan about 1-2 weeks ago ;_;
This was the first one that I have ever seen for
sale (and I have only seen 2 others in personal
galleries).  It actually went for a reasonable
price, I thought considering the rarity it would
have gone much higher even though it was a small
image, I think it was kind of a fluke that it
didn't.  Here's the link to the auction so that
you can see :

http://www.rinkya.com/cgi-bin/twview.pl?URL=http://page7.auctions.yahoo.co.jp/jp/auction/g6710930

I guess your best bet is to keep an eye out on
Yahoo Japan, here is the link to the Trigun
section (sorry if you are already familiar with this):

http://list5.auctions.yahoo.co.jp/jp/2084035213-category-leaf.html?at=true&alocale=0jp

And keep an eye out on the Keep-Track section of
this site :

http://www.cels.org/db/keep-track.pl?cat:1

for dealer updates.  You might also try asking the
dealers with Trigun cels about maybe keeping a
look out for you, but I can tell you that he is
probably on every Trigun collectors wishlist and
if one does show up it will probably be pretty
pricey :(

Take care,
nesuferit