I don't want you to hold out false hope, so I'll put it bluntly. You can find rental units maybe in Texas, where enough demand exists, along with local companies that manufacturer large smokers.
In So Cal, your chances are pretty much zero. It's a topic that comes up from time to time in the BBQ forums I use (I'm a BBQ competitor), and nobody I know personally is willing to rent out their competition / catering equipment to someone they don't know and trust. Too much risk and not enough reward from an individual owner's perspective.