Microsoft often has special pricing that they do not advertise, or seemingly actually try to hide, this is one of them. 

If you meet the following criteria you are eligible to buy Windows 7 for just $29.99

1. You must have a valid e-mail address at an educational institution ending with the domain suffix .EDU (ie, leina@contoso.edu) OR have a valid email address at one of the educational institutions listed  AND
2. You must be a student at a U.S. educational institution and must be actively enrolled in at least 0.5 course credit and be able to provide proof of enrollment upon request.

Maybe you know someone who qualifies and might be amenable to offering their copy to you.  This is probably deal you can find anywhere, so get the cheapest price on windows 7 right now.

For a retail offer I would recommend purchasing Lowest Price Windows 7 at Amazon. By purchasing online you will avoid sales tax, and if you sign up for their Amazon Prime offer, you will won't pay for shipping either.

Now if you go to school, college or have kids who do, you can take advantage of academic pricing, which is below retail. Academic Superstore is one of the most popular and agressively priced sites online. You will need to provide a copy of a report card or current student ID, and you can buy not only Windows 7, but many other software products, sometimes at up to 75% discounts.  You can find Adobe products such as Photoshop and Dreamweaver for up to 80% less than you'd find at your local store.

So there you are, as advertised, you now know where to find the lowest price on Windows 7.

Are you hesitant about moving to Windows 7 before it's tested in the marketplace. Buy Windows Vista after June 26th, and you are eligible to upgrade to Windows 7 for as low as $9.99. So buy your Windows 7 from Amazon, install it as soon as it comes out (Oct 22, 2009) or wait a few months until Microsoft releases the first batch of patches (They always do) and then install it.

If you buy Windows Vista after June 26, the can still get the Windows 7 upgrade for $9.99!
It's a special offer by Microsoft called the Windows Vista Upgrade Option. You won't see this offer advertised much, it seems to be a bit of a secret actually. My guess is that it's legally advisable for M$ to offer a special offer for a reasonable amount of time to customers who purchase an operating system that is now no longer the latest software. Of course MS won't make much money on $9.99. Luckily you now know this fact as well. You can get Vista, order your Windows 7 upgrade, and either install it right away, or wait a few months and then install it and all the inevitable patches as well.

Here's the secret. First buy copy of Vista (Again I recommend Amazon, but it's good no matter where you purchase), the offer is valid on items obtained between June 26, 2009 and January 31, 2010.

Here are Microsofts terms...

• This offer is only for customers who purchased eligible software at retail.
• Not valid for original Equipment Manufacturer (OEM) or volume license customers. Windows Vista Home Basic, Windows Vista Starter Edition, and all editions of Windows XP as well as ROEM SKUs, COEM SKUs, Academic SKUs and volume license SKUs are not eligible for the Windows 7 Upgrade Option Program.
• Offer valid in participating countries only.
• All orders must be accompanied by proof of purchase in order to be processed.
• Select the appropriate product(s) on the Windows 7 Upgrade Option Store to complete the order.
• Customers limited to one Windows 7 boxed product for every one corresponding qualifying Windows Vista boxed product purchase.
• Offer applies only to initial purchase or acquisition of Windows Vista Product.

The website for this offer is

https://om.one.microsoft.com/OPA/htmlpages/TC/Win7upgrade/Win7UpgradeTandC.htm