There is no 1 special shop, there are a few across the country. The ones I know are in SoCal. I haven't personally had any service done by them yet, but these reputable ones are off the top of my head:

http://rsmachines.net/

http://www.blueprintmfg.com/

http://laskeyracing.com/shop/shop.htm

http://www.hamotorsportsusa.com/


Just do research on them and find the one that suits your needs.