In New York, I have been using Mark Tashijan's shop, and they seem pretty reasonable and very knowledgeable:
Formula Motorsports
http://formulamotorsports.com/
3216 37th Ave
Long Island City, NY 11101
(718) 482-0515
When I lived in Palo Alto, I always thought Tony Heyer was great:
Heyer Performance
http://www.heyerperformance.com/
2362 Old Middlefield Way
Mountain View, CA 94043