There are three options for buying Diamonds that you can choose from:
1. You can buy Diamonds in the Bank. Click on the green "+" button by the Diamond bar in the upper-right corner of the screen to open it.
2. You can take advantage of limited-time sales that offer Diamonds at a discount.
3. Recharging your Cornucopia Fountain is the best deal you can get! It yields 120, 200 or 340 Diamonds daily, depending on the charges you choose to purchase.
If you haven't got the Cornucopia yet, complete Rangi's quest and treat him to Mangoes in exchange for the Fountain.