The only aftermarket spare tire for the Tesla vehicles, including the Tesla Model Y (all configurations of Model Y), is from Modern Spare:
Modern Spare: 2020-2022 Tesla Model Y Spare Tire Kit Options
A full size wheel and tire (even the 19" Gemini wheel and tire) would take up most of the rear trunk area. Even the Modern Spare mostly fills the upper trunk luggage area (the Modern Spare won't fit in the large lower storage bin.) None of these would fit in the front trunk (frunk) unless you modify the vehicle and remove the frunk bin.
Most Tesla vehicle owners rely on Tesla Roadside Assistance, AAA if they have a flat tire. It is advisable to carry a 12V tire inflation pump and a tire plug kit so you can quickly plug a punctured tire without needing to have your Tesla vehicle towed (requires a flat bed tow.)
If the Tesla Model is equipped with the tow hitch option there are aftermarket tire carriers for the tow hitch. You could carry a full size spare on trips using the tow hitch method but this would probably impact the aerodynamics of the vehicle and reduce your range. Still, people who travel deep into national park areas off of main roads may feel the need to carry a full size spare wheel and tire using the tow hitch.
So far no tire manufacturer is offering a run flat tire with the needed tire load, tire index rating: 255/45R-19, Extra Load (XL), 104 Y.
If you decide to purchase the Modern Spare order well in advance of your trip as it can take months to receive your Modern Spare.