I second everything that NRHA said. Beyond that, you'll just have to start looking at studs when you start getting closer to the time it comes to breed her. Any stud we mention now may not still be producing when that time comes...and there may be a whole new crop with different bloodlines.
What I would do is start diligently watching reined cow horse events, see what bloodlines are winning (and pick the winners that you like) and look into lines like those to see if they might match your mare.
It also depends on what level you're looking to do with her foal. If you're just looking for something that can get the job done and win at local/fun level, then you could likely widen your search to certain ranch bred studs. Many of them are also suitable for what you're wanting because they carry a fair bit of cowy breeding and also all-around performance breeding that would make them good reining horses too.
Not really sure where that came from
. OP didn't say anything about western pleasure.