where does one obtain a wind tunnel?
The sky above you makes for a pretty good wind tunnel. After the rocket motor burns out, the acceleration you can measure on board is directly proportional to the rocket's aerodynamic drag. As long as the speed is high enough to get significant deceleration, the drag calculations you make that way can be quite accurate. Here are some Cd vs. velocity plots I calculated from some flights in the last 2 years, using a Parrot altimeter:
A 29mm rocket flying a conical nosecone and an Apogee 29mm nosecone, both with G80s
A 38mm rocket that went almost 16,000 feet on an I600, using a Von Karman 7:1 nosecone
The increase in drag near Mach 1.0 really stands out in these plots. And notice how much lower the increase is for the Von Karman nosecone, which was invented specifically to produce the theoretical minimum wave drag for low Mach numbers.
The drag coefficient was calculated using the measured rocket diameter, the as-weighed landed mass of the rocket, and altitude-specific atmospheric density. A drag coefficient of 0.3 is definitely achievable without polishing; these rockets were smooth and a little shiny, but not mirror-finished. What some may not realize is that it's a lot easier to get a low Cd with a fat rocket than it is for a long, slender rocket, since the Cd for rockets is normalized against the frontal area. A long skinny rocket has a lot more skin area compared to its frontal area, and that shows up in the Cd. A short, fat rocket doesn't have much drag for its frontal area, which is what the Cd is all about. I have flown a very short, matte-finished rocket designed for the F10 motor that had a drag under 0.3; it was 0.27 to 0.28 if I recall correctly.
But like cjl said, the drag is the 1/2 x area x the Cd x the velocity squared, and each of those factors is just as important as the other. Some people think the aero drag losses go down if the rocket breaks Mach, just because the Cd goes down. In reality, the drag force at 1200 mph was over twice what it is at 800 mph for the 38mm flight, above, even though the Cd was lower.