This isn't really on the north shore, but there's this Mexican market on Pulaski right south of i-55 called Pete's. They had a ton of them not too long ago. If you're desperate, you might want to try there.
The new-ish Lewis Fresh Market on Grand Avenue in Waukegan has fresh poblanos, and a large selection of other Mexican items. From Wilmette, take US-41 to Grand Avenue (IL-132) and go east, past Green Bay Road.
Poblano peppers are pretty common. Yes, Whole Foods carries them. I've also seen them at Marketplace on Oakton in Skokie, which has an excellent fresh produce section. My local Dominick's even carries them.