how to do it
https://www.youtube.com/watch?v=2zsWRSEnxaA
where to get the parts
www.fcpeuro.com or www.skandix.de
approx price is $120usd + shipping for kit... e.g.
https://www.fcpeuro.com/products/volvo-tailgate-strut-replacement-kit-stabilus-31217640kt
in that link are all the part numbers