In the UK, or anywhere in fact, I believe the set sold by Accurate Armour is the cheapest.
http://www.accurate-armour.com/ShowProduct.cfm?manufacturer=0&category=12&subcategory=57&product=516The most expensive is WW2 Productions set, but that does include sprockets and other extras.
Losing the original ones is probably the best thing you can do with them in many modellers minds. They are pretty bad and are best depicted covered in mud, but you'll still have to use the pins or thread methods to replicate the track sag, unless of course you do spring for after market track.
Check here in this thread to see how it can be done with standard track.
http://www.planetarmor.com/forums/showthread.php?t=5193If money is an issue put up a wanted advert, after all with so many disliking the originals and buying after market they will have the originals lying around spare.
Alan