There are two sorts of sail numbers, the ones that boat companies and boat owners put on the sail to give the boat a racy look. These can be purchased from almost any sailboat shop or marine supplier. Lots of owners match the number to all or part of their hull ID number.
Then there are the numbers assigned by sailing clubs and organizations to their registered members as member identification.
If you join the United States Sunfish Class Association you will be assigned a set of numbers like the ones seen on so many Fish. You don't need to race to be a member. $35 annually - sail number included (I believe)
United States Sunfish Class Association
P.O. Box 300128
Waterford, MI 48330-0128 USA