Brian,
Many companies offer commercial GL policies, which is called the standard market. When agents or brokers can't find a company that will write a policy because the risk is unique (and in my case, sometimes they are just too unfamiliar with what I really do) they will turn to a speciality market, also called surplus lines, or non-admitted market. Errors and Omissions falls into specialty coverage.
A good place to start is at your State Department of Insurance website. You will find agents who are licenced for commercial insurance, and also who handles surplus lines. You can also call other inspectors or inspection companies in your area and ask which insurance agency they use, and contact them for a quote.
The next best advice is to really work with your agent so that they have a good understanding of the business (whatever it may be). That said, When I put my business insurance out to quote, I give the agent a 3 or 4 page document with photo's, certifications, and a resume. then follow up with them BEFORE they visit with the underwriters to make sure they don't have any questions.
~thirdeye~