Hi Jeff,
There are so many different sources for BTs, I think it depends what you are looking for. I think a lot of people start off with Shadows tracks and then diversify.
There are a number of very good sources, probably the most central source (they sell the lot, pretty much) is Leo's Den (
www.leosden.co.uk), (post free in UK), but you can buy from the individual producers of the BTs.
On some of the individual sites there are demos of the tracks, to give some idea of what you are buying, these are some of my favourites, in no particular order -
Shadows Workouts Vols 1-14 (
www.shadowsworkout.ucoz.com)
UB Hank
Dave Robinson (
www.pastmastersmusic.co.uk)
Roger Allcock (
www.vintage-sounds.co.uk)
Roger Paulsson (
www.nostalgiband.se/roger/backtrax.english.htm)
Pete Korving (via
www.graceland52.eu)
There are others, of course, and I'm bound to have missed some of them, there are some from Australia too.
The BTs from Europe, namely Roger Paulsson, and Pete Korving tend to be more expensive, but are very good quality.
There are also free BTs available and a couple of the good ones are -
Goran Tangring (
www.goran.tangring.com)
mathieu (
http://users.telenet.be/mathieu.laats/default.html)
A google search will reveal a lot more.
There is also a very god site for Tabs -
Tony Clout (
www.tabman.co.uk)
There are also free ones available on web.
That should be enough to get you started . . . hope it helps.
Regards
Brian