Imp Locations
If you are collecting beads for the Imp Catcher quest, the path between Lumbridge and Draynor is a convenient location for hunting the Level Two Imps that drop beads. Other locations where Imps spawn include south of Falador, north of Yanville and on the Port Sarim dock. There are two Imp spawning locations in Varrock, one near the back of the palace and the other close to the west wall.
Trading for Beads
Colored beads are not sold in stores, but you can buy them from other players via the Grand Exchange. The price of beads varies greatly; as of August 2011 they cost between 150 and 230 coins.
Using Beads
The Imp Catcher quest requires that you collect one bead of each color and give them to the Wizard Mizgog in the Wizard Tower south of Draynor Village. Imps drop different color beads randomly, so it's likely you'll end up with several duplicates before you have all four. Beads can be used to make Magic Boxes more attractive to Imps when you're trying to catch them. You can also sell beads; the Wise Old Man buys them for 16 coins, but you can make much more money selling them on the Grand Exchange.
Imps in the God Wars Dungeon
The Imps that appear in the God Wars dungeon are different from those at Level Two. They are Level Seven Imps and do not drop beads.