I didn't see how small you were wanting to go but this is from Arms and Equipment. It may be intended for an animal but couldn't hurt to run past your DM.
Shrink Collar: Rather than resizing to fit its wearer, as most wearable magic items do, this half-inch-wide metal collar studded with tiny jewels resizes its wearer. When worn, the collar grows to 2 inches wide, and the jewels enlarge to cover most of its surface. Regardless of his or her original size, the wearer shrinks to Small size, becoming around 3 feet in height and weighing no more than 60 pounds. None of the creature’s other attributes change. Unwilling targets get a Fortitude save (DC 11) to resist. If the save is successful, the collar snaps and is ruined.
A creature with Intelligence 2 or lower must succeed on a Will save (DC 11) or become panicked by the transformation. Animal companions do not have to make this save, nor do creatures that have the effect of the collar explained to them beforehand somehow, such as through telepathy or speak with animals.
Caster Level: 5th; Prerequisites: Craft Wondrous Item, reduce; Market Price: 10,000 gp; Weight: 1 lb.