As I presume you are new to your 3, you might want to look thru the various files I have collected re the air suspension system per the link below. I note that one has over five thousand views and another over four thousand, hence I guess others regard them of value as well.
You 3 has what I call air springs - that means a balloon inside a "tin can" and thru which a conventional shock absorber runs. While one can replace the shock and reuse the other parts, it is normally best to replace the whole assembly.
Regarding parts, (any parts), I would stay away from eBay or Amazon.
They might work OK for a Chev, but for your 3, you need advice first, and then more importantly, the correct part. For a 3, that is not so routine. Used, I would purchase from people here on the forum; new from the likes of Atlantic British, BP Utah or Lucky 8 - those kind of sources as their people know Land Rovers.
The dealer, yes I purchase some parts from him here locally, but they know the newer vehicles better than the older units - also the prices are full retail - not always the best.
http://www.disco3.co.uk/gallery/thumbnails.php?album=3778
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