If you are a Costco member, or know one, check them out. They've had nice prices on Blaupunkt before. Otherwise, Ebay (if you're comfortable with Ebay), and Sounddomain.com can be good.
Crutchfield can be good too, because they often throw in freebies like remotes, and they always include wiring harnesses and dashboard fit kits free of charge, along with customized installation instructions for you specific application. Their shipping charges are reasonable, and based on the total price of your order, so no surprises there either.