I'm not sure if the local pet store would buy the baby fish that come out of the pond, I haven't asked them about that.
Goldfish are fairly easy to raise in a pond setting. All you have to do is provide them with the right stuff to lay their eggs in and you'll get baby fish, as long as you have 5 or 6 that are mature enough for breeding.
In the past, we've gotten baby goldfish from the pond most years, but not this year. We had to put a new liner in the pond this year so the pond didn't get setup until the middle of the summer.