You can cut down an old class spinnaker for a shy kite or buy a used one from a smaller boat.

But the class spinnaker is already too small for typical sailing conditions in most areas of the country. And even when it's windy the boat carries a masthead spinnaker in windard-leeward racing just fine. And in light or medium wind a masthead spinnaker perks the boat up quite a bit. So, in my opinion, a masthead kite would be a better investment than a shy kite, in my opinion, if you are not racing one design.