Hi Barkingdog and thanks for your input.
My vet does suspect Belle has some Border Collie and it is possible that when she does go after the cat, which doesn't happen too often anymore, it's a herding reflex because it only happens if the cat makes a sudden swift move (which also doesn't happen that often, he's old). However, Belle just goes ballistic if she sees a cat outdoors, or a squirrel. She also has a tendency to walk with her nose to the ground (hunting?) which I have to be constantly correcting.
My vet wasn't able to recommend anyone. I've been given 2 names of trainers by people who've not had any personal experience with them. And the university veterinary hospital where I've been going for the past year has also not "personally recommended" any trainers.
I know swimming would be great. I've heard there's a dog pool in the city somewhere, which I'm looking for now. I've also been told that mental exercises can really tire a dog out, and Belle has shown a little bit of potential in obstacle course at the dog park so that's an eventual possibility, I just need to get basic commands down really solid first. She doesn't recall because she's too distracted by critters.