I have seen Andes brand chips at Target in mint and peppermint flavors, I think. I am not sure if the Manhattan store carries them, but you could give them a call to see. The E117th Store does have fresh grocery, so of all the Targets, they may carry them.
For a better mint chip, I would order Guittard chips from King Arthur Flour or a similar company.
Guittard Green Mint Chips - 12 oz.
Dylan's Candy Bar does have dark mint bars, which could be chopped into chunks for cookies. I am sure that you could also find mint chocolate bars at many other stores, even grocery stores, such as Whole Foods or Trader Joe's. Just be sure that it's a mint crisp bar, lest you have a gooey mess with a filled mint bar. Whole Foods does have a mint crisp bar in their house brand, 365 Everyday.