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Walmart sells one for BBQ's but with the fat and grease I went with a piece of plywood under it to "soak" up the mess that I make. Works great for concrete but if it is wood you will need to move it so the area will "air" out or dry after a rain or snow.
Are you looking to protect it from a mess or from potential fires? Grills on wooden decks should have a flame resistant mat under them, especially charcoal or wood burners.
Also a good idea to mount a fire extinguisher near the cooking area.
Raven's grease soaked plywood is a fire hazard if it's under one of his Webers.