The reason why you don't find worlds with already made stables is because everyone wants different buildings and different layouts. There are loads of equestrian worlds out there (look at Carmen Raymond's creations or by Heather Reinhart) and equestrian stables (also by Carmen as well as many others) that you can place on those worlds. If you are looking for a world with a stable designed into it, then you are going to have to ask a good soul to make you one (I've made a world for a friend once and let me tell you, never again... it's way too much work) because sadly the only time those will happen is when someone makes their own world with their stable on top and with all the time they spent on it, they are unlikely to share it, especially publicly... because nobody wants to have their stable they made especially for themselves being used by everyone else.
It could happen if someone made a stable and world and then decided they didn't like it. They may be willing to part way with it and give it publicly but again... since the stable was originally made for them, the amount of CC and number of EPs used will probably scare away most people.
Other than that, I know there are some show worlds with buildings, but they aren't meant to be private stables... just show grounds... I know Heather Reinhart at least made one already for show jumping years ago. Not sure that will work for you but that's the best I got.