Rochester Big and Tall on 52nd street is probably the best for this. They carry lots of mid-to-high end designer stuff that's miles ahead of the expandable collar and waistband crap at your local store.
kidkim, I was referring to the ubiquitous "Casual Male Big & Tall" stores that George Foreman promotes. No matter his size, any guy can do better than that. I haven't been to any other Rochester stores besides the NYC one, but I would assume they are similar, perhaps with a lesser selection.