I find very hard or impossible at all to communicate with salvage yards if you are looking for something else than a headlight or hood.If are there phisycally it is different,but phone or email is a nightmare.
I cant help you where you can locate such a seats,you need to search.I was thinking to try to fit the multicontour seats from my 06 530xit to the X5,but didnt even have time to check if they are similar.But X5 seats should be plug and play becasue the seat modules are part of the seat.The seatbelt pre-tensioner you can replace,it is held by one bolt.