Where to get Windows 7 for self-built computer

You can buy an OEM version many places.  newegg.com (where I buy most of my PC parts) is one place: http://www.newegg.com/Product/Product.aspx?Item=N82E16832116758&cm_re=win_7_pro_oem-_-32-116-758-_-Product

CompUSA is another: http://www.compusa.com/applications/SearchTools/item-details.asp?EdpNo=5213934&Sku=M17-7306

No processor, motherboard or any other computer hardware comes with any operating system, you buy that separate. You can install 32 bit version of sonar in 64 bit OS, I recommend you buy 64 bit OS. If you build your own computer, when you buy the processor you need to know exactly  what memory speed you can use.
Example..if you go with i7 920 processor the best memory speed to use will be 1066 mhz. I'm not sure about the 1333 mhz but I know that 1600 mhz or higher will give you tons of trouble.

Just do a nice research about it.....

In the US, most places will sell an OEM version without any special qualifications. 

With currently available hardware building a PC is not the hassles it used to be.  I can remember friends going around for weeks getting a new PC to work or work properly.  Now, as long as a component isn't "zapped" (e.g. static electricity) during the build usually they will power up and work the first time. 

Three caveats with OEM Windows7.
1. It is not licensed for use by builders of single machines. This represents a change from previous Windows licenses. Whether that will be enforced is a question. Since the license is intended to put it out of reach of DIY builders and no longer blesses the purchase if it is done with a component, most resellers have stopped requiring you to buy any hardware before they will sell you the OEM. Almost none will explain that the license is dodgy.
2. You are not guaranteed that you will be able to move the OS to a new computer if you upgrade or make major changes later. The OEM version is the one computer assemblers sell with their pre-built machines, and MS policy is that those versions go with the machine and die with it.
3. The OEM does not contain both the 32 bit and 64 bit versions of the OS as does the retail full version.
OEM versions tend to sell at about half the (pretty outrageous) retail price, so the savings may be worth the inconvenience and uncertainty.

I got this one at Comp USA. Both 32bit and 64bit versions included. Did a clean install on a brand new hard drive because my existing OS drive was about to die. Perfectly legit and easy. Breathed new life into my three year old laptop. Now I can stall until the Black Friday price drops on the i5's and i7's.