Heavy duty rubber bands (such as those that hold broccoli bunches together) are great for managing cords, attaching gear to your go-bag or backpack, and other uses. All you need to do is cut them from an old bicycle inner tube with a pair of scissors as shown by Instructables user dent244.
The name ranger bands comes from Army Rangers who would use the inner tube bands to tie down gear such as flashlights and knives so they didn't move or make noises while the troops were deployed. They're now used by many backpackers, preppers, and tinkerers wherever you would normally use a rubber band. The butyl rubber that makes up inner tubes should also last a lot longer than most modern latex "rubber" bands. You can also use them as tinder to start an emergency fire if in a wilderness survival situation.
If you're not a cyclist you should be able to score a popped inner tube for free from a cycling friend or bike shop. One tube should provide 10-20 ranger bands easily.
Ranger Bands (Rubber Bands on Steroids) [Instructables]