Where can I source Mexican chilies in the UK? 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?
Mex Grocer has an excellent selection: http://www.mexgrocer.co.uk/. I've ordered chillies once from them in the past with success.