Hi an welcome! Not sure on value, but your best bet is the inevitable eBay auction. Failing that a small ad in Railway Modeller or similar publications or you could, if you don't mind a lesser price, try your local model shop should they deal in second hand goods.

You could also try advertising in some of the model railway forums, but please introduce yourself first. Personally I like this one and I also use New Railway Modellers....Google will bring up the URL.