The best place i have found is
www.pricegrabber.com ( i think that is the page might have spelled it a little wrong) it finds a handfull of merchants selling the product you are looking for then lets you choose from the list of the best priced ones, and it highlights the best price and figures in the tax and or shipping for the item also for you. I have seen a 1 gb MS pro duo go for as low as $112