I'm going to agree with Waterpond on this one. Koi get way too large for a fountain unless you are talking about a very large fountain. For instance, I bought a small butterfly koi last year in August that was around 3 inches (minus the tail), at the beginning of April this year he measured almost 10 inches long. He grew 7 inches in less than a year.
Here's how I do it. I have a small fountain that I built containing a total of about 250 gallons of water, when I first buy my koi (if they are small), I use the fountain as a 'quarantine tank', I keep them in there for 2-3 weeks. The water is shallower and I can monitor them more closely. After I have fed them various medicated foods for whatever nasties they may have, I move them to my large 3250 gallon pond for permanent residence. I have to keep a very close watch on the water parameters in the fountain though because with the small amount of water, the temperature can get very high on a hot sunny day and I have to add fresh water to bring it down. The fountain is set up just like my pond, live plants, bio-filters, waterfall, and spray fountain. Lots of oxygenation for when the water temp climbs. I even have bubble stones and bubble bars on hand like people use for aquariums for really hot weather.
If your going to use a fountian, make it a temporary thing and monitor everything very closely. If your going to keep koi, I would recommend atleast a 1,000 gallon pond. The smallest I would go for a single koi is 500-700 gallons to give it plenty of room to move around at its full potential size of 3 feet.
If you like koi, don't let building a pond disuade you from keeping them. It's well worth every bit of the trouble and cost. They are truly wonderful fish to keep.