I'm not sure what tune you're referring to, but 
this page outlines the music featured in that 
episode:

http://rfblues.aaanime.net/Sessions/Music/music06.
htm

And not to discourage you, but it could be a bit 
of unreleased music. On the other hand, if it is 
unreleased music, it could appear on the 5 disc 
limited edition boxed set coming out in June. So 
there is still hope!

Hope this helps!

--rg

PS. Here's a Cowboy Bebop cel for good measure.