@boomerang,
Hi, i've been looking for one of those lamp mechanisms for weeks on google, ebay, from european lighting retailer (all lamps I've seen that use this mechanism are from there), etc with no luck at all, that would be the bad news, the good news is I do have a lamp that has exacty what you need, and it is not inside a bubble in the middle of the cable, it is inside the junction box? cover. It would be basically the same as a retractable keychain but twice the diameter, twice as thick and made with some metal parts instead of all plastic, I think it even has wire instead of a cord. So in the unlikely case you get one if this lamp mechanisms it will be someting that will probably last decades, the lamp I have is vintage and again, it is not the bubble mechanism mentioned in a previous reply, it would be more like a heavy duty keychain mechanism. Hope this helps