I'd say start looking into GSD rescues. From your description, the right GSD does sound like the correct choice for you, and going with one from a rescue will allow you to have a better idea of the temperament and capabilities of the dog. Just bear in mind that many GSDs now do not have much herding drive anymore. I was very fortunate with Bimmer, but he's not full blooded . . . A GSD/Australian Shepherd mix might be perfect for you.
And an Australian Shepherd comes very close to suiting your needs as well.