There are many ways to obtain rare weapons which are more powerful than normal items.
When you kill bosses, sometimes they will drop a weapon with an "Elite" tag
You can buy them from the shop with diamonds
Use tickets to play annihilation mode and try getting them there
Buy crates using diamonds
Finish off special zombies
Only 10 lucky survivors will be able to acquire these rarities during the final battle of Survival Mode
Equip yourself with elements that increase the chances of finding better loot
Instantly kill bosses with bombs to destroy them in one shot.