The Association for Driver Rehabilitation Specialists was established to provide driver education / driver training and transportation equipment modifications for persons with disabilities through education and information dissemination. It has instructors and evaluators who can certify your ability to drive an accessible and modified vehicle.
https://www.aded.net/

Funds & Grants for wheelchair-modified vehicles
https://www.braunability.com/help-me-buy/financing/#grants-funds

Access Paratransit - Santa Clara County & Interactive Paratransit Site
http://www.vta.org/getting-around/paratransit/accessibility-paratransit-service

Metropolitan Transportation Commission for the Bay Area
https://mtc.ca.gov/our-work/plans-projects/other-plans/coordinated-public-transit-human-services-transportation-plan

Public transportation options by county for the San Francisco Bay Area
Alameda
http://accessalameda.com/

Contra Costa
http://www.mobilitymatterscc.com/

Marin
http://marinaccess.org/

Napa
http://www.ridethevine.com/ada-accessibility-0

San Francisco
http://www.sfparatransit.com/general-info.htm

San Mateo
http://www.samtrans.com/Accessibility/Paratransit.html

Santa Clara
http://www.vta.org/getting-around/paratransit/accessibility

Solano
http://www.solanomobility.org/

Sonoma
http://sctransit.com/paratransit/

Department of Rehabilitation Mobility Evaluation Program
https://www.dor.ca.gov/Home/MobilityEvalProgram

Wheelers Accessible Van Rentals
http://www.wheelersvanrentals.com

Accessible Vans of America - Nationwide rental of Accessible Vans
http://www.accessiblevans.com

BLVD - Marketplace for wheelchair accessible rental vans as well as those for sale
http://www.blvd.com/

The National Mobility Equipment Dealers Association (NMEDA) is a non-profit trade association dedicated to expanding opportunities for people with disabilities to safely drive or be transported in vehicles modified with mobility equipment to fit their specific needs.
http://www.nmeda.com/

For-profit companies specializing in vehicle modifications for individuals with a physical disability
Ability Center:
http://www.abilitycenter.com/
Access Options:  
http://www.accessoptions.com/
Lifestyle Mobility: http://www.lifestylemobility.com/
Mobility Systems:  http://mobilitysystems.com/
Mobility Works:  https://www.mobilityworks.com/
Modesto Mobility Center:
http://modestomobilitycenter.com/
Sacramento Van Conversions & Mobility: https://sacvans.com/