I use Facebook pages a lot but also equine now is a good one. The only problem is sometimes you have trouble getting in contacts with the owner of the horse.
I love to use a variation since everyone posts on different pages
I love dreamhorse.com and Bigeq.com also equine now is great too
For the us I like equinenow
Depends on what country you are from... or if you wanna ship a horse from another country! There are also lots of groups in Facebook