Phil,
They are available now and then, but the pros today won't really use them. First, you need to have generally 4 or more since once you get the paint set up to spray, you want to shoot all the covers at once. If you have one mask, you can't easily do that.
Second, you don't get a crisp line with the mask, not as crisp as you do with hand masking.
I've had wheel covers painted by 3 different painters and none would use the mask. They all preferred to mask by hand. Some used tape, and others used this liquid mask that paints on and peels off.
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America