Polespear sling bands are cheap and easy to make. They're also relatively cheap to buy, but they are usually assembled with a metal staple to hold the sling cord, which is a little unsightly and can clink loudly on your polespear if its metal.
You can buy bulk sling material from most dive shops. You can also use surgical tubing or rubber tubing. Surgical tubing isn't as good as commercially made band material, but its often cheaper and sometimes easier to acquire. If your local hardware store doesn't carry it, you may find it at a laboratory supply store. In a pinch you can make a fair sling band from a bicycle inner tube.
Cut the sling to a length where, when fully stretched, the sling is stretching 300% its length. Stretching a sling further will give more power, but the returns are diminishing and the sling will wear out faster.
Thick nylon line works well for the cord. Tie a big knot in each end and force them into the hole in the tubing. Needle nose pliers work well for this. You then need to tie some more nylon line (thinner this time) over the tubing so the knots won't come out.
The knot is as easy to tie as a shoelace. Eventually I'll get a picture up, but its so easy that I'll try and explain in words alone. Lay a piece of nylon string (about 18") straight across a table or floor. Put your sling across the middle of the string so that one of the ends (where you will make the knot) is perpendicular to the string. Tie a loose "shoelace knot" over the sling (just the first part of the knot, not the "rabbit ears"). Do not tighten the knot yet. Flip everything upside down so that the loose knot you just tied is on the table (or floor). Tie the same shoelace knot again, but this time take one of the free ends and wrap it under the string of the previous knot as you are making the new knot. Put some gloves on and pull with all your might (or use pliers). The knot will cinch down tightly and neatly. Do another such knot if you feel it is necessary (its isn't). Cut and burn the ends so that they don't fray, and drop a little glue on the knot if you have some lying around.