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Among the chiles commonly available in U.S. supermarkets, Anaheim or New Mexico are probably the best substitutes. A mild jalapeno would probably work, too, though you might want to start out using less of that than the recipe calls for, and increase to taste.
Heh. Must've been typing at the same time.
Anaheims are a common sub for poblanos. Depends what the recipe is.
If it's for stuffing, you can even use some bell peppers or cubanelle peppers. If it's an ingredient in something, use a small jalapeno (stripped of ribs and seeds).