Michael's & Hobby Lobby carry them. I don't know if they carry this "cheaper" model--just the electric handle and 2 blades--but I know they carry the Creative Hot Marks/Walnut Hollow set. You can find those in the wood crafts area, usually. WalMart also has the larger set--it contains some little extras.
forgive me for thinking out loud...I wonder if an electric knife would work?
hmmmmmm.....I usually use a razor without too much problem. I discovered foam core about 8 years ago, and started making bird houses out of them...never realizing I would be using them for cakes one day. But a round is a little tricker than a square.