I had a hunt around and I have to say you're right, it looks like it's not available anywhere. It appears to be Volvo part 9146761 / Bosch part 0 280 160 554.
You might be able to find a used one, but I haven't seen that many 10V 850s around or heard of any being parted out. It does look like the part was used on other 2.5L Volvos though so you may have some luck. @iceton1975 might be able to help as he works for Berry Motor Group in Melbourne who have probably the best local supply of used spares (and he's quick to get parts in the post).
If it was totally impossible to get hold of one, I think your only other option would be to use a generic adjustable unit, but that would come with its own set of headaches as you'd need to figure out a way to mount it and so on.
