Weaver makes a miniature nylon horse halter that fits my 33" boys pretty well. It is not adjustable; I do not like adjustable nylon halters. That pesky buckle under the chin gets in the way of the lead rope ring. It is not a cheap halter, usually costing about $15, but it is well made. I also like their lead ropes. They are a good weight. I do shorten them, though, as they are too long. Not every place that carries Weaver will have the miniature halter, however.
One thing that keeps a halter from fitting miniatures is the strap under the jaw is too long. It allows the noseband to go down too low on the nose and you cannot tighten it enough over the poll to make it fit. I hate a halter that hangs down on the soft cartilage of the nose.
Anyone care to count the misfitting halters hanging in the barn? The ones you don't get rid of because you need them when you are desperate.