On-site vs Right-of-Way (ROW) contractor
First step is to familiarize yourself with how your wastewater plumbing system works and the scope of work that needs to be performed by a contractor/plumber. You will also want to familiarize yourself with the District property line cleanout inspection process and/or the District disconnection inspection process that is needed to meet your District permit requirements to understand what type of contractor/plumber will be able to help you.
A contractor/plumber used to repair or install components on your wastewater plumbing system located on your property should be properly licensed for that work by the State of California Contractor's State License Board. When your scope of work requires sewer construction on the District sanitary sewer system in the public right-of-way or District easement, a District registered contractor is required to perform the work.
To ensure you are being quoted a good price, you should elicit quotes from at least three different contractors/plumbers. You may check the local Better Business Bureau, online reviews, or a friend or acquaintance for any recommendations. Be sure to read the contractors/plumber's contract before you sign it to ensure that the contract meets the needs and requirements of the scope of work that is to be performed.