You dont need to purchase them - Saints & Sinners have a great h'cap calculator at
this site
The way handicaps work from a starting point of zero, if I recall correctly, is that you have to shoot 3 scoring rounds and your average h'cap is then taken from those rounds. Subsequent scores are then averaged against your running average throughout the season.
For example - you shoot a 65, 64 and a 63, giving you an average of 64. This is then your running h'cap. If you then shoot a 62, you will lower your h'cap by 1 point to 63 (taking the average of 62 and 64). If you end up with a decimal point figure, its rounded up to the nearest whole number.
At the end of the year, your carried forward h'cap is averaged over your 3 lowest h'cap scores of the year.