Hi Laura,
www.ponyonly.com usually has a pretty good for everything but show halters. For show halters you can go through the same dealers you use for mini show halters - almost all of them make custom sizes and will make the halter to your measurements. Spazkat should be able to help if you are looking for a Shetland sized in hand bridle for showing Moderns - I have no clue where to get one of those! LOL