Pretty much every supermarket sells it, it costs more than the draino but if you're dosing small amounts regularly, then it lasts quite a long time.

Kelly, we rented out our house for a few years, and if a plumber had been called out (for anything really, other than a toilet blockage caused by flushing the wrong sorts of things, or a kitchen sink blockage caused by putting too much food/oil down there) I would have needed to pay for it. Having hair down your drains is normal, it happens as part of normal use of the property, so it's worth checking with your rental agent about getting some plumbing maintenance done. Getting it done earlier rather than later could potentially save damage & clean up & 24 hr emergency plumber costs, kwim?