Wonderland Plaza is home to Adam the Clown, one of the many psychopaths prowling Wilamette Parkview Mall. Defeat him - using a large chainsaw yourself actually works pretty well - and Frank will have access to the small chainsaw, which is better than the standard chainsaw in many ways.For one thing, you can actually carry the small chainsaw in your inventory, meaning you won't have to drop it before picking up another weapon.
The other cool bit about the small chainsaw is that you can greatly extend the amount of time the weapon lasts by picking up three books - Entertainment, Criminal Biography, and Engineering. It's one of only three weapons in the game that are affected by holding three different books. The small chainsaw will continue to spawn at the entrance to the ride in Wonderland Plaza after you've beaten Adam.