EquineNow, DreamHorse, and Canter USA are my favs :)
Bigeq, Facebook, warmblood sales, dream horse, etc.. I usually just search for what I’m looking for and it shows me the top websites to go to.
Warmblood sales is really good but the horses tend to be more expensive on that site.
EquineNow is my favorite. You can also look on Facebook. There are so many different groups that you can join specifically for buying/selling horses.