If you're not familiar with cotija cheese you're in for a treat. Cotija is a firm Mexican cheese that doesn't really melt, but instead adds a nice chewy, salty bite to your dish. If your grocery store carries it, you can find it next to the sour cream and ricotta. If you can't get your hands on it, you can use your favorite salty/firm cheese instead.
You can use whatever enchilada sauce you prefer in this recipe. I use H-E-B(R) That Green Sauce when I can get my hands on it, but feel free to experiment with your favorite homemade or canned sauce.