There are several ways you may get extra resources: 

Resource Tiles in the World
There are three types of resource tiles available in the World View, Crystal Tile, Mithril Tile and Gold Tile. Note that the gold tiles can only be found in the War Zone. 

Raid Skeleton Village in the World
These villages are guarded by skeletons. The higher level they are, the more resources they may have in possession. 

Raid players of enemy factions
You may find enemy factions' players in the War Zone or in their homelands. Note that you also get honor points by killing units or destroying buildings that belong to enemy factions' players.

Guild gifts
Joining a guild will provide you with many benefits, one of them is to gain access to the guild gifts. Not only they may contain all kinds of resources, but also gems and special perks.