ada compliance

I was curious if anyone was aware of a good, trustworthy company that does ADA compliance audits. We are rolling out a whole lot of changes to our site and would like to go through a formal process of having compliance reviewed. I figure there has to be people out there but wanted to check with all of you first.

The semantic structure of a discussion can be ruined if a comment includes code for headings that are at a higher level than the title of each comment. In discussions in Drupal, the title of the page is an <h1>, key regions are headed with <h2>s (which sometimes do not appear on screen but are available to screen readers to define regions that are obvious on visual inspection), and each comment begins with an h3. So if any heading is added within a comment, it should be no higher than an <h4>, right?