What can you use in your drains to clear hair clogs if you have a septic system?

You should be able to use most all the OTC drain cleaners on the market, just read the label, and follow the directions. Don't go overboard with them. If you're referring to the larger pipes, heading out to the septic tank, then a snake is about the only reliable form of cleaning. This is usually performed by a professional, either a plumber or a company like Roto Rooter, however, there are rental companies around that rent the equipment to do the job, just be sure you know what you're getting into before going this route.