A step down transformer

Buying a step down transformer ($80~200+ USD) will cost more than buying a new US spec "Penni Press" sandwich maker for $20~50 USD.

Just use it as it is without a transformer and expect the "Hot Sand Maker^(sic)" to toast things faster and maybe wear-out slightly faster.

The first time you plug it in at 120V, I would put your Peanuts Sandwich Maker on a non-burnable surface. I doubt there is any real danger. I have used a lot of Japanese 100V devices in the US without any fires.