You get this along as you keep playing. It'll pop up as long as you keep killing zombies!
There's not much else to it.
If you want a faster strategy, then go ahead and farm the Ace missions on hard mode.
To select hard missions, beat his triple coin mission and return. If you don't see a hard mission, back out and talk to him again until you do.
Play his hard missions to get the most kills.
In Woodlands @ level 6 you kill ~86 zombies during each run, with both of his missions on hard difficulty.
If you run his missions 10 times, then it's garunteed to pop before you leave Woodlands.