Have you tried
http://www.volvopartswebstore.com?
https://www.volvopartswebstore.com/products/Volvo/2009/XC90-44l-8-cylinder/Starter-motor--exch/1156428/36000040.html
They're not cheap. We had ours done on the S80 V8 just after we bought it and before we picked it up. Seems they're a bit of a mongrel to get to, which is a lot of the expense.