The imiv seems popular, I have a similar unit that pretends to be a cd changer, and has a 3.5mm stereo jack, so I can use my mp3 player or friends phones or whatever.
Iirc it was about 80 quid, which sounds a lot but the standard head units is pretty good, so replacing the head unit wou mean losing or severely complicating standard features like steering controls etc.
I got mine mail order from Canada of all places. Check out some Volvo forums for ideas, Volvoforums.org.UK seems pretty good, plenty of other tips and advice too.