Windows 7 is available for preorder in Microsoft’s own store, but you can also pre-order Windows 7 through Amazon. Here are the links:
Windows 7 Home Premium (Upgrade):
Windows 7 Professional (Upgrade):
Amazon is known for fast and reliable shipping, but I assume that Microsoft has also done everything in order to make their own shop run smoothly. Therefore you could also give Microsoft’s own store a try and see if they offer some of their products for cheaper!
I will add a list of products and a price comparison table to this post as soon as I have some time for it.