Last time I checked the camber bolts run about $40-$50 a set for the rear. I'll have to check my stock struts I have sitting at home, but the camber bolt should go where the bottom of the strut mounts to the back of the brake assembly. Its a rather large bolt and should be easy to spot. The camber bolts you will buy are ecentric, where as the stock bolt is just straight. You should take the car in for an alignment so that they can set the camber for you as it would be really hard for you to line it up yourself. After I lowered my car the first thing I did was take it in for an alignment and Subaru was able to get it with factory specs. I have no abnormal tire wear and i've been lowered for 3 years now.
I would, it is going to be very hard for you to get the correct camber setting yourself. The only other way is to do trial and error, adjust the camber bolt, see how its wearing, adjust it some more until you get it right, but you could easily go through a set of tires doing that and I don't suggest it.