How do I get spades to unlock building plots?

Spades are required for the last set of plots in the bottom right corner. You might get some through various items under the Bonus tab when you click the Newspaper that says Events Today in the top right corner. You might also get some for various events. They also show up sometimes in packs or the Special tab under the Store. The last two options cost real money, but are probably the only way to get the required 2,000 spades without waiting a long time.

Building plots are HQ level locked in the game. The first ones that you may be able unlock are the surrounding areas of the war game room. Each of the sections are gradually opened as you reach certain HQ levels (HQ levels 6,8,10) of the game. As you reach these specific HQ levels, you are entitled to buy each of the spot for 15,000 paint. When purchased, the will be available permanently. Hope this helps!