For DNS and hosting I would think whoever you are using (Network Solutions, GoDaddy, 1&1) would provide support for most issues there.
For Postfix, the
email support list is absolutely awesome. If you follow the guidelines, provide what is asked for they can typically get you through any hurdles with Postfix itself.
Outside of that you can search for consultants in your area. contact your local Linux Users group. a Local ISP (We have several local ISP's and security companies that focus on OpenSource solutions and will provide paid Linux support)
You can post the details of the consulting gig HERE in the
LQ Job Marketplace (or ask a forum admin to MOVE this message to that forum.)
There's also a
Linux Consultant List you can search by location..
and of course theres always Google.
Google Linux Consultants