I think the way I would approach is that the bearing is just that....a bearing.
Most bearing companies will treat it as that and measure the internal and external sizes, type of bearing, load and speed rating and then find you a suitable replacement (which may have a higher revolution speed or load rating) but I could almost guarantee that there will be something available, just not listed as a replacement Nissan wheel bearing.
Try these guys: http://www.ashleypower.co.uk/