They are Tamiya (TAM70100) which gives you various cogs and tension pullies plus a selection of different lenth tracks that can be connected together.
I bought the last lot from here as they are cheap but you can get them in the UK.
http://www.buy-model.com/itemdetail.php?Id=862&FormZoneId=204&ShippingType= Surfacehttp://www.buy-model.com/itemdetail....pingType=Surfa ce
Some other Tamiya sets have motor and gearboxes included but they obviously cost more.
I then add some of the sticky rubber on the outside to improve the grip.