Most souvenir shops where they also sell the Disney coffee. Though IIRC--I can only remember hot chocolate and flavored coffees...not recollecting seeing Disney tea--but it would be stocked with those if they have it.
I see it in the kitchen shops (DTD and Liberty Square perhaps), souvenir shops (the big one on main street--more towards teh front of the park..they have a whole section with mugs and such) and we also saw this stuff at Contmeporary, but not as big a selection.
In England---you will find authentic teas and not "Disney" teas--though they are quite good.