Where should I get Windows 7?

OEM versions are mostly bought on-line (ie newegg). NOTE, you can not upgrade your MB and re-install the OEM version. You can if your MB dies and you have to replace it.
Upgrade Version, about the same cost as OEM. But you must own a copy of say windows XP - either that is currently on the computer you are upgrading, or a copy you OWN but IS NOT on another computer - If this is the case you do NOT have to install windows XP then do the upgrade, as you can do a "clean" install using the upgrade version. Just have to do a few extra steps, SEE pg 2: http://www.maximumpc.com/article/howtos/how_use_your_windows_7_upgrade_disk_fresh_pc?page=0,1

Full versions, way too expensive and very little cost savings buying on-line vs downtown

Windows 8
Revised their OEM Lic to allow upgrading MB and re-using the OEM copy.
Pretty much split on Love it vs hate it.
uSoft, is coming out with a windows 8.1 (Windows Blue) that addresses a number of the problems (HOPEFULLY adds back a start menu, But there a some 3rd part add-ons that provide a start menu ie Start8.
Windows 8 upgrade, Not sure if it will allow a "Clean install", if it does NOT then Skip the upgrade version.
 
Windows 7 (Except for Family pac) can only be installed on ONE computer at a anygiven time. This means that:
1) If computer dies, you can re-install one your NEW computer, This includes the OEM version.
2) If you upgrade your computer, You can re-install UNLESS it is the OEM Version. 3) You can ONLY re-install the OEM version IF the MB dies, You can NOT upgrade the Mother board and re-install.

In the above cases, You may get a message stating somethink to the effect it is not a valid copy, BUT you just need to call Microsoft and explain. They will issue you a New Key code.

If You just need to re-install on the machine that it was already on, ie your HDD died, Then that is NO problem.
 
To continue what RetiredChief is saying.

It is all about the licensing and some about the code.

An OEM copy of Windows 7 is always tied to the computer on which it was originally installed. You can not transfer the license to any other computer.

Using the Upgrade version of Windows 8 consumes the previously installed OS's license. You can not install the previous OS on any other computer.

The OEM License for Windows 8 works differently but you can still only install it on one computer at a time.

As far as where to buy it? Amazon, Newegg, Tigerdirect etc.

Sometimes the OEM version of Windows will be bundled with the purchase of some hardware like a harddrive, memory, CPU, motherboard etc.
 
So Windows 8 is better? Will it run any faster or slower? Also, I got Bioshock Infinite and Tomb Raider for free and got offered Windows 8 for Bioshock Infinite but he might have been lying because he's done that before.