I can't comment specifically about the elongated C7 transverse process treatment providers, but I would think most TOS docs/surgeons should be capable or know of someone who is.

Here is our list of Docs & PTs - California -list complied approx 2006 so there may be new ones - but the offices/ staff should be of help w/referrals/names if you contact and ask about elongated C7 transverse process -
http://neurotalk.psychcentral.com/post703-1.html