There is a store in Vista that has experience with at least one Boxster, and I was happy with the results. It is called Costco
. They had a very competitive price on Michelins too. I don't know if they just do balancing.
The real problem it seems to me, where ever you go, is if you get someone careless who marks up your rims. I make it a practice to get assurances that my rims won't be marked up and they didn't mark them at Costco. Cann't say the same for Discount Tire.