Cotija is everywhere here. Even our local Kroger sells it. I buy it in random weight wedges...
...at our Mexican grocery store.
It isn't sour or bitter to me! To each, his own, but I find it tastes heavily of cream, but has a dry and crumbly texture. A dry feta or parmesan would work fine in the recipe, I'm sure.
It is born of street food, so a bit of 'experimentation' is normal, I think.
It's funny. 50 years ago, you couldn't find fresh cilantro anywhere...
Michael