First off, be careful about what contract your client signs, especially with small companies such as First Health/Coventry or Empower Chiro. These types of companies discount everything from United Healthcare (regardless of your contractual status) to Workers Compensation claims which could be losing your client up to 75% on many claims.
Most major insurance comanies have a provider directory and you can enter the specialty and zip code to get a list of par providers, even some of these subnetworks have sites like this but remember there are clauses in many of these contracts that allow these 'networks' to give the same discount to another network your client is not contracted with.
For example, Private Health Care Systems bought Multiplan which means if you are contracted with Multiplan you are obligated to discount all united healthcare claims, and as a non par provider you only get aprx 60% of their 'R & C".