I recommend the 1N5817. It's a 20 volt, 1 amp, Schottky rectifier. If it's used "across the supply" as Mooly says, it'll protect as well or better than a silicon diode like 1N400x. If it's used "in series with the supply" it will definitely work better than a silicon diode; 1N5817's forward voltage drop is lower, so it'll keep the effects pedal circuits working another 2-3 hours longer when the battery is nearly drained.
Avnet Express sells them for CAD 0.0382, in quantity one: (link). They have 4,400 of them in stock & on the shelf, today.