These should only be obtained from a physician. Ice is a great way to reduce pain and inflamation. A good way to apply ice is to freeze water in a paper cup. Peel the cup back to expose the ice and then use the cup as a handle while gently rubbing the ice over the effected area. Ice is particularly good for the first couple of days.
Some people find that it's useful to continue ice treatments beyond that.
Others find that the ice treatments make their backs tight if they continue beyond a couple of days. Heat, especially moist heat, can be useful. However, it should not be used for a couple of days after injuring your back or after aggravating a current injury. Regardless of the timing, if you feel worse during or shortly after heat treatment, stop doing it. In the later stages of a back problem, I find that my low back muscles get tight.