How can I calculate polar curve?
Here are some other methods (excluding CFD and computational ones) for building up a polar curve:


Wind tunnel testing, one of the earliest methods, still one of the most applied and explored one.
Flight testing ("dynamically scaled" model or a 1:1 real prototype)
Empirical methods (e.g. Hoerner's books on "Lift" and "Drag")
Reverse calculations; i.e. if the performance charts AND the engine deck (force model) is published or known, then the flight performance chapter of the aircraft operations manual could be used, at least to a certain confidence level, to guess about the polar curve for different configurations and conditions.
If you know a few speeds of best endurance/best range glide, you could get an idea of the CL/CD at those speeds, and if you know the mass of the aircraft, the polar curve can be plotted.
Asking the manufacturer certainly could help (if not a model of commercial competition)