Endrega is a 2-strength Ranged Combat unit without any ability. Think of it as cheap filler that helps you hit the 22-unit minimum. Monsters decks also have a useful perk: after each round, one random unit of yours stays on the battlefield.
No merchant sells it directly. It comes as a random reward for beating generic gwent players, so merchants, innkeeps and villagers are your best bet. It’s from the base game and part of Collect ‘Em All.
Generic merchants stop handing out new cards once you’ve beaten a certain number of them, so spread your games around. The book A Miraculous Guide to Gwent (patch 1.20) shows how many cards you’re still missing per region.
