Everyone knows about Tasca and DW Volvo, but I found another Volvo dealer's website that seems to have even cheaper prices:
www.VolvoPartsLisle.com
I put in a couple different part numbers and the prices were lowest on volvopartslisle. The prices between all three sites were within $7-$8 or less, but that can add up on a large order. Plus, it can't hurt to have another source for cheap Volvo parts 
I should note that I have not ordered from this site before, so I can't comment on customer service. DW Volvo is the clear winner in that regard.