Well, tortillas are not a problem, but those chilis would probably be harder to find. I know I can find preserved jalapenos. Tomatillos I had no idea what was until I just looked them up 5 min. ago
One question; you can't be making much of a roux if the the meat is only supposed to have a dusting of flour? Not that I would want a heavily thickened sauce here anyway, but it puzzled me. Perhaps it's just a difference between what we respectively think of as a roux - I looked it up some time ago, and it looks like the "American way" is to put a great deal more oil in than I have learned to do. So how much oil would you add here, given the very small amount of flour?