Depending on your location, a good source is to ask the existing marketplace and surrounding gas firms who they use. Many times an engineering firm specializes in a specific formation or basin, such as Western Slope Colorado. This gives them an advantage with regulatory agencies and region specific requirements. Case in point, We deal mainly in gas process, gathering and transmission in the central / western US only. CO, NV, NM, OK, AZ, UT, WY. Good Luck