If you are grinding for money to unlock this I found a really amazing spot that you can go to to earn money pretty fast.
In the level Santa's workshop, after you are able to enter the shop on the conveyor belts that have enemies coming out of them. They never stop coming out. Endless supply of enemies for you to kill, as well chaining kills nets you extra ,money. I did this on the second conveyor belt right after you flip the switch to bring down the elevator in the beginning area of the workshop. Other conveyor belts have enemies on them as well, but this is a smaller belt and real easy to kill them right when they spawn. Also note they drop ammo as well, so I would advise to sometimes let them come out of the spawn so you are able to grab the ammo.
Do this for as long as you need to.
*Note: Make sure you finish the level after you get the money that you need to purchase the items in store. DO NOT QUIT OUT OF THE LEVEL.