I don't care for Koi in small ponds as they do make a mess of the plants but every big hotel on the BI has a water feature with Koi and they breed like crazy, you might be able to find some offspring from whomever maintains the ponds. Their growth would be constrained by the size of your containers so they would never get very large. I would bet there are a number of pond maintenance services in your neck of the woods and they may even have a source of goldfish.
