Well... most shops are located in the same area, just get down there, walk a few shops, ask for the prices and get the one that gave you the cheapest one (local set and not grey set), test the lens out if everything is okay, then buy lor.
Normally I will send them email, then ask for availability of the lens, before making my way down to the particular shops that have the lens and selling at the cheapest price (local only and not grey), and buy.
Usual suspect are - Cathay Photo, SLRR, MS color (not very attractive pricing recently though), TKfoto and John3:16 (although they are abit anti-Tamron, not sure why)... and recently Click! which quote me one of the most competitive price so far, I think one of this days I need to make a trip down and look see look see.