Where do I get the peace shield in last shelter?

You can buy peace shields in a few places. And you may also get some as rewards for daily and CoZ activities. The quickest way to activate a shield is to tap on your base building in your city or on your city in the world map. Then tap the City Buffs option.

In the list of buffs Select the peace shield, and then you can select the duration you want. 8 hours or to 3 days. If you have one already you can activate it. If not then you can buy it with diamonds.

If you want to buy a peace shield in advance you can buy them for diamonds in the item shop. Tap on the Item menu then select the Store tab. The shields are under 'Buff' . Or you can find some in the Alliance Store to be bought with alliance contribution points. Check with your alliance R4/5 members if they are not listed.