Personally, Mathis Rowan always seemed like the most prominent Targ-supporter in the Reach. Various minor houses less connected to Highgarden could support them as well, but I like the work Lord Varys did demonstrating the breadth of Tyrell power in the Reach. Tarly is a wildcard, but you'd need to give him a pretty good reason to flip, as he's presently in a great place.
Jeyne Westerling's hips always really threw me. I was really sure Jaime had the wrong person, and a little part of me still wonders if GRRM didn't tell an out-and-out lie, though that's not really like him.