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I would imagine that some office chairs are fully assembled at the time of delivery, especially the larger ones. But for the more economic sized chairs, they will come in boxes in which the consumers will assemble at the desired place.
They do in fact offer one the chance to buy their chairs in actual stores. However, they don't work with many larger chains so you should check their page at http://www.hermanmiller.com/Where-to-Buy , which will tell you local stores which offer them.
You can purchase an L shaped computer desk pretty much anywhere! You can look at Target, K-Mart, Fred Myer, Office Depot, Office Max, Staples, Uranus, Mars, Neptune, and Pluto. Those are the places I would check.