Looks like the TCS Mini Pro class will be going to 21.5 B/L next year. Don't know how many on this thread race the TCS series, but just posted it as information. TCS usually goes with major manufacturers so all of the speedos and motors should be available from the large hobby shops and from your LHS.
That was the knock on the system used in Australia in that trying to find one was like trying to fins a "speakeasy" in the days of Prohibition. To my very young friends and non USA friends there was a time that alcoholic beverages were illegal in this country. Thus the term Prohibition and an illegal, underground bar was termed a "speakeasy". How products are distributed are a little different in the various locals. What might be widely available in one area may be vastly different in another country.
Seems like a good solution cause the experiment with 17.5s was not totally successful. They were too fast and were too much for the spec tire we were expected to run. They worked better in the "outlaw" races which allowed the smaller 55mm tire which have a lot more "grip". The other problem was they were very hard on the drivetrain and reduced universals to junk in half the time and outdrives were notching on very little run time.
Sorry, "grip" is caveman for traction or the coefficient of friction between the tire and the track surface. Come to think of it, so is "loose" and "slick" in other contexts. But then, I drink my coffee from a "mug", like women with "boobs" and "legs", a fan of "big" cars and "sprint" cars without "wings" on "dirt" but realize it was way too dangerous, and think that F1 is the ultimate form of racing. Altho, my favorites were the "midgets".