It took me A LOT of packs, but I finally got a single map pirate zombie. They can come from the 1,000 coin packs which i recommend since you can buy a ton of these at a time. They are rare, same with most of the plant consumables that were added. I'd recommend playing gardens and graveyards if you already have your 15 gnome bomb wins as you can easily get 10k+ coins on that game mode.
*EDIT* Got the achievement. Took me probably a hundred 1000 coin packs, but I finally got the barrel zombie. A little too rare if you ask me.