Equiery is specific to MD. If you're going to Chantilly, then that's a better bet. There's a plethora of horse stuff there so you'll have to be more specific in what you want to do. H/J, eventing, western, trails, etc. If you're there, head to Dover Saddlery on rte 50 and they can lead you in the right direction. There's a few places outside of DC. A friend has her horse really close to DC.

Average, for horses with some experience at this level