Insurance is my day job.
California laws always throw a monkey wrench into everything, so I'm likely just talking out of one side of my mouth, but here goes: What you want is short and long-term disability insurance. This will cover you in the event of injury that prevents you from working for a period of time. AFLAC is a popular example...the duck commercials.
1) How you're set up will largely determine how to move forward. Do you have a DBA and tax ID#? In other words, are you the sole proprietor of an actual business with all the forms and pomp and circumstance that goes along with being a "real" business? If so, you'll be looking at commercial rates, which are typically higher.
2) If you're just self-employed without a business, or a freelancer who relies on contract work, you'll need to show some proof of income that you're looking to replace with the insurance. Tax returns, receipts, all that kind of jazz will be needed.
3) Choosing a longer "elimination period" - aka how long it takes for you to receive the benefit/payout of the insurance - will be the biggest determining factor of your premium. If you choose a short period (like a month), expect to pay much more than a longer period (like a year).
4) Work with someone local. You can buy this stuff anywhere, but don't do it over the internet. Go talk to an actual agent who can get to know you and your situation and make sure you get what you need.
Hope this helps!