If you did not already obtain the information you needed, please let me know. I can put you in contact with someone in Sonoma who arranges for a lot of inspections of wells/septic systems. While she is not an inspector herself, she runs an affordable housing company that contracts out a lot of this type of work. I am sure she could put you in touch with one or more inspectors.
Best of luck,
An excellent place to start would be the Permit & Resource Management Dept. for the County of Sonoma:
Here's a chart showing current well and septic fees:
There is a very good book that gives an overview of well and septic systems called "Cottage Water Systems" by Max Burns and distributed by Firefly Books in the US. It is most important that you work the right professionals to inspect them and who is also familiar with building restrictions on properties that contain well and septic systems so that you don't purchase a property with some inherent limitaton to expansion based upon the percolation (perc) capacity of the ground or the production of the well. Approx. 70% of the homes in Sonoma County are on well and septic. If the seller doesn't pay for your well and septic inspections, plan on spending a good $1,000 minimum, in addition to your pest and contractor's report, for these inspections, and go to the county offices to read their files on the property and talk to the planners and well and septic people so that you understand the property and any possible limitations. Another thing about wells and some septic systems is that you often need power in order to pump water unless you have a gravity fed system so in a power outage you will often need to be prepared with a generator or just some storage. I'd be happy to refer you to a local agent with experience in this subject. Good luck in your research.