The reason why you can't own slots until they are 25 years old in California is because of a state law that blocks slot machine gambling except in Indian casinos. Machines older than 25 years are considered "antiques" and "collectibles" and are excluded.

There are current Game Kings available for sale at various stores in Vegas. They may not be the newest models, but even the newest models are sold to private citizens. I believe IGT maintains a store for Nevada residents in Reno.