Not sure this is Range Rover equipment - as it has sirrus support, but maybe someone has added it as a retrofit. From the quick info I read on bimmernav it's market specific too, USA and Japan I think.
Either way, in answer to your question, Google BM54 brings up many, many results with a few online retailers. Mainly the very top Google result, failing that eBay?
Sitting here grinning. I'm sure this is one of those language-barrier probs. I think what he meant was 'would someone sell me one at a reasonable price because the ebay prices are too high'.
That said, Greek Ace I just happen to have been watching these prices myself. If you check on past auctions, the selling prices on ebay .de, .co.uk, .fr, and .com are all about the same: roughly €150. A 'buy it now' from a vendor costs around €100-150 more. Logical.