Rumble Card: Penguin Stampede

Rumble Card: Penguin Stampede is sold by Ulaani in Deeprun Tram and  Tiana Nevermorn in Brawl'gar Arena for 500x Brawler's Gold. It summons the Penguin Stampede encounter.

Source
The item can be bought for from:
 * Card Trader Leila in Bizmo's Brawlpub
 * Card Trader Ami in Brawl'gar Arena