LOTS of cheap bells these days with Christmas sale. My dollar store has a leather strap with large sleighbells attached and a big brass ring that will fit over a doorknob. Or like mentioned above , you can hang it from a screw-in cup hook by the door.
They are a WHOOPING.... 50 CENTS right now at the dollar store after Christmas sale. If you can't get the kind with the leather strip and door knob ring, alot of dollar stores have other sleighbells and hanging bells you can use.
Bells on a string can also be easily used
