For over thirty years SuitAbility sold sewing patterns to make riding apparel and horse equipment. The owner retired and now only a limited number of remaining patterns are available from this source while supplies last:
Field's Fabrics
Amazon.com store (search for SuitAbility equestrian patterns)
Versions of some paper patterns are available in downloadable PDF format in the Etsy store
SuitAbility Epatterns. 56 different PDF patterns are available for western and English apparel, horse clothing, saddle pads, stable gear, and horse costumes.