Ant138 in his current Little Walter post mentioned a site called www.shoblues.com as a good source for backing tracks. There is a small fee $1.50 for downloading a track.
An 'Off the wall' style track can be found on track 9 HERE on the site and you can listen before you buy.
I love little walters instrumentals....it is where he excelled....played like coltrane and/or charlie parker....but to try and imitate him????it cant get any tougherIMHO
I have used www.shoblues.com and recommend his stuff. For something a bit more polished (and a fee) try the following for great Little Walter backing. No "Off the Wall" however.