I've traveled to Mexico and the US and seen different chilies available there, but at home in the UK (or even worse, in my fiancée's home of Finland) I find it very hard to find Ancho, Poblano and Chipotle chilies.
Oaxaca Restaurants lists a lot of interesting-sounding chilies, but I have no idea how to source them.
I have once found dried Chipotle chilies in a supermarket, but at that time I was not entirely sure of the right way to rehydrate them (and I'm still not).
Where should I look for North American type chilies? And if I only find dried, can I use them instead of fresh?