In New Mexico, it's easy to find such chiles. Some specialty markets in other states will carry fresh New Mexico chiles, but those are only going to be available in-season, during harvest.
Be warned: Those of us outside of New Mexico can be in for a twisted maze of false - or at least regionally different - labeling. While most New Mexico chiles - including the famous Hatch green chile - are visually indistinguishable from Anaheim chiles, and Anaheims are grown year-round in California and thus available fresh for not-perfect but workable substitute in recipes, actual Anaheim chiles are very often labeled as "poblano" chiles in markets all over the place and, of course, poblanos are then labeled Anaheims. What's a New Mexico chile-lover to do? Go by how they look. Those things above? Those are New Mexico chiles. Look for them.
The other option - far preferable in my mind - is to buy canned Hatch green chiles. They are available roasted and whole (usually about 4 chiles to a can) or roasted and diced, perfect for tossing into stews or stirring into sauces. For those willing to put in the time, do as those in New Mexico do and roast up a big batch in-season and pop them in the freezer to pull out whenever you crave that perfectly spicy and flavorful kick New Mexico chiles have.