It all depends upon what you want and what your tastes are. Here are a couple of sites with lots of pictures of wheels, as well as the wheels on cars:
http://www.wheelenhancement.com/index.php?t=Wheels
http://www.wheeldynamics.net/
Tire Rack and Discount Tire Direct also sell wheels for Boxsters, and if you're going to buy tires anyway, you can get them delivered with the tires already mounted. Both of their websites let you put the wheels on a picture of a Boxster.
You can also check on eBay, but you really have to know what you are buying, ask all the right questions, and be sure that you're dealing with someone reputable (check the feedback).
Have fun!