I cannot confirm that a normal X5 35i/50i suspension can fit an X5M/X6M. Using KW's website, the part numbers are different. You'd also have to deal with an EDC/active suspension trickery, which there are HW devices that/can do that...
That said, B6, B8 and B12 suspensions are easily found in the UK for the X5 35i/50i.
http://bilstein.suspensiondirect.co....-xDrive50i.jsp with USD pricing and shipping info
Good luck on your search.