Security is an important part of this line of work. If you work for an agency, the agency might be able to provide you with one. Nevertheless, the best bet is to actually hire one of your friends or someone you can trust, who lives nearby in case of last-minute shows/cancellations.
If your friends already know what you are doing, and accept it, sell them on the idea of helping you out. Tell them that they can make great money on the side and don’t even need to do anything besides driving and keeping a watchful eye. A fair amount per show is $20 and some percentage of the tips. Plus, they’ll get a free show!
You can always go to yellow pages, or even craigslist.com for security options, but be sure to check reviews and references to make sure you can trust the company/security guard.