Erie won't have any wrecked 2012 S60's in stock - there are 5 or so JY's in the country with a new S60 right now and I'd hardly count on a wrecking yard to give you all of the wiring and parts required for something like this.
Tasca Volvo has excellent pricing on new parts, but their shipping can be expensive. Are you a member of VCC? VCOA discount gets me 15% off parts at my dealer which usually is the cheapest and most convenient way to do it.
Also, you will need the dealer to program the sensors so you'll have to pay them either way.