The stock answer is Ebay but there are for sale threads on RCU and most other web sites like this.
Before I spent Ebay money for the CG kit I would just buckle under and buy the Great Planes 60 size Extra kit. Nothing wrong with it at all and a very good build. There is a difference in how they fly so I like the CG kit better but there isn't enough difference for people to be spending the Ebay money I have seen being asked. Still, you never know when the super deal will come up on some of these sites.