You want to get 14mm x 1.50 as that's the measurement for the lugs. Looking online, seems Tesla charges $105 for a full set, but I think these $15 ones on Amazon would work just fine: https://www.amazon.com/dp/B083LN55M3/ There are some $50 ones out there with the fancy chrome finish if you'd prefer.

Tesla's lug nuts are stainless steel. They really shouldn't get rounded off unless someone is using the wrong size socket, or is going crazy with power tools and really overtorqing them. So long that someone knows what they're doing, it shouldn't cause damage to your car. That said, sounds like damage happened under the previous owner so you might as well replace the lugs.