The SEO people who know what they are doing? They won't cold call you, they won't respond to ads. Actually many of them don't need to advertise and in some cases there's a waiting list because (just by word of mouth) they're in full demand.
If you need professional help, your best bet is to hit some of the better known, reputable search blogs and forums, read up and see who seems to know what they're talking about, and approach them that way. If they can't help you, often they'll give you a good referral to someone else (reputable) who can. At that level, there's not a lot of fighting over business, because there's enough for everyone. So people are happy to refer.