It's all in the question, I am looking for places where I can rent a bike, in Palo Alto or Mountain View. And whether I have to bring my own gear or I can rent a helmet as well.
There are several! I guess you'd want a list or the 'best', so I've tried to accommodate that by linking to the Yelp list of such places in Palo Alto.
The currently 'top rated' one there is Dubbelju Motorcycle Rental, at 274 Shotwell St San Francisco, CA 94110 United States
Phone number (415) 495-2774