Newegg. Amazon. Tigerdirect. Ncix.
Windows 7 will always be 80-100. 80 on sale. if you see it cheaper it's a scam.
You REALLY should consider windows 8. The windows 7 OEM license can only be used on ONE system/motherboard. Ever. Then you have to buy a new license. Windows 8 does not have a "retail" version. The OEM version you get can be moved to as many machines as you want. The only restriction is it can only be on one machine at a time. windows 8 has a horrible UI, but that can be fixed with classic shell, which is free.