Bonus stamps, or points, are earned by making purchases in the Gem Shop, and are found by navigating to the Gem Shop and then tapping the "Bonus" tab.
In this tab, you'll see how many stamps you've earned and your progress towards each bonus tier. When you've earned enough stamps to unlock a tier, you will then be able to claim the rewards for that tier. So, for example, it takes just 5 stamps to unlock the first bonus tier. Then, it takes an additional 95 bonus stamps, or 100 total, to unlock the second tier (as seen in the above image), and so on.