find a "road force" balance machine such as a Hunter 9700 as Jim suggested. Check this website for a zip code search. My closest Discount Tire just installed one a month ago, which is nice to have close by... and cheap too. $19.99 a tire. The high performance shops were nearly double that when I called.
http://www.gsp9700.com/pub/search/findgsp9700.cfm
Other readers of this post should search too to find one nearby and make a mental note of it! Boxsters seem to be very hard to get into wheel balance and keep there. This machine really got my boxster smooth and vibration free after many, many balancing sessions at 4 different places!