Tenchi also sells posters and wall scrolls. There's an outlet in Ang Mo Kio and Toa Payoh.
ebay has everything, or try Amoeba music if you live in L.A. or San Francisco...also fye has alot of posters.
You could look for brocantes and vide greniers (kind of giant Yard Sales) in the area. There are lots of these in May/June and September. Select ones with lots of persons selling their stuff (100+ exposants) and arrive early (8.30 am maximum) to get ...