Expert witness testimony

How do you get an expert witness from the California DMV? Does anyone have a direct contact or department within the DMV that handles this specifically? If I can’t secure an expert witness from the DMV, do you have any suggestions on how to find independent expert witnesses in matters related to driving laws? Additionally, what is the process for getting a subpoena to compel a DMV employee to testify if I have difficulty with the first two approaches in finding an expert witness?