call you local metal fab shop. here's what you need to order:
A36 grade steel
3/8" thick
14.25" x 16"
beveled edges all around
should run around $60-65 completed
once you get it home, wash it with soap and water and dry it off. once dry, oil both sides with a high heat cooking oil. then place on a grill and burn off the oil to build a carbon layer. repeat the oiling a few times to help build a protective coat on it. once it's seasoned, you're ready to use and enjoy it.
pizzas, smash burgers, grilled veggies, whatever you want to cook on a flat top.
after use, and when it's still hot, use tongs to hold a washcloth or bunch of paper towels and spray water on to wipe it off. it's very hot so DON'T touch it. after wiping it down, just re-oil that washed surface and reseal it. enjoy!