Windows OS Download

I just got a new rMBP yesterday and will be using it for work. I need to run some Windows-based software on my computer, so I purchased Parallels in order to run Windows. I'm looking for the best place to download Windows 7 or 8, but am having a difficult time finding a good price. Does anyone have any suggestions as to where I can find a decent price for Windows 7 or 8? If so, what should I be paying for them? I am a part-time student, so I am eligible for any student discounts, however I do not currently have a Windows license so I cannot purchase any updates.

Thanks!

 

I think I'm answering my own questions by Google searching, but I'm still having some problems.

It looks like I want to get a Windows System Builder license. However, all options that I can find are physical DVDs that get shipped to me. Since I have the rMBP, I have no optical drive. Is there anywhere I can buy this that will let me download it online?