Horseclicks.com I found that bigeq.com is more on the expensive side
There are specialized groups. What are u looking for? FB has those groups.
Facebook groups! Join one that is your discipline and go through the posts daily.
I found all of my horse from Facebook pages!!
Retiredracehorseproject.org or canterusa if you're looking for a thoroughbred or an inexpensive project
proequest.com is where i found my horse and i had very good luck
www.horseclicks.com and www.equinenow.com
Dreamhorse.com is always my go to