I have found 5 sources of practice tracks - 2 are paid, the rest are free.
The Chord Perfect tracks have 3 sections avalable free as samples - if you want all of them, they have asked for at least $10. ChoraLine uses synthesised tracks (quite good in my estimation) and costs £7.99 with the Overton discount (enter OVERTON in the voucher box) - you can sample on the website before purchase.
All the free sources have their parts synthesised rather than sung. The Musipedia tracks are available on YouTube. I don't rate them highly, but you can see the music as the track plays. Cyberbass have a handy player on their site (much like Chord Perfect). John Fletcher music require you to create a login but once you've done that, you can download all the parts from their dropbox account. Your part is differentiated in the same way as ChoraLine but is not stereo.
My recommendation is to try Cyberbass first (unless you're a fan of the Chord Perfect sung tracks). Personnally I'm going to use John Fletcher as the tracks are slightly better and I can put up with downloading Dropbox files.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. this is the bottom of the page