There's a seller on German eBay (www.ebay.de) who sometimes has secondhand LGB sound units. His identity is skyppy1 and I've bought stuff from him. Postage is not cheap, but for me it still worked out a better deal than buying in the UK. There is nothing of that sort at the moment, but may be worth putting on your watched sellers list, If you have one.
grootspoor also have one for less than RPP (180 € or £163) but once you have added their frankly exorbitant payment fees and postage you'd be better off with the ebay one listed previously (£146 + P&P) LGB 65006 Dieselok Sound Nachrüstsatz (2x52x)
The ones I got from the seller specified in post #4 were £88.65 each plus shipping of £14.83 - I think they were listed at €99. I know they are used, but I've had no trouble with anything I've bought from abroad - yet!!!!!