I would definitely recommend Facebook, and any form of social media really :^)
Maybe ottbs? They need homes and they are always available for dirt cheep
Okay maybe not hunter/ jumper but there are also eventing horses and such
Facebook groups are awesome, maybe try hunter/jumper horses on the facebook groups
Advertise, usually on Facebook. Show photos and videos of you riding. Imagine you are an owner, what you would want to see in a potential rider?