Many brands sell ground cloths (or “footprints”) specifically made for tent models and sizes. Go to the brand’s website (or REI) and search for the name of your tent plus the word “footprint,” and the right one should pop up. Similarly, if you’re buying in a store, just ask one of the sales associates, and they should be able to direct you.
You can also buy more universal or generic tent footprints on Amazon. They typically come in one-, two-, and three-person sizes and won’t fit your tent’s shape perfectly. But they do the job, and they’re usually cheaper.