You can change out the back and leave the front with no problems, but you'll notice more of an improvement if you change all of them out at the same time.
Try
www.eshocks.com - I paid about $260 for a set of 4, and at the time I bought anything over $200 or so was free shipping. They shipped quick and at a pretty good price.
Edit - they are very easy to install. I spent about 2 hours, working slowly, to change out the whole set. The back set is easier to change than the front, but neither one should be a problem. It's easiest to take the wheel off of the front to change it, but the back you can just crawl under and change.