What Windows 7 Product Keys will work for Bootcamp!
At least the following versions of Windows 7 will work find on bootcamp.


Home Premium
Professional
Ultimate


all work. In order to obtain a key for one of these versions, You will need to find an ISO and a Matching Product Key for the same version of windows. 

Microsoft offer downloads of Windows 8 from the Microsoft Store. If you're set on Windows 7, then your options are limited.


If you have a MSDN account, you can download an MSDN ISO from there.
You can purchase Windows 7 from a reseller (Amazon, NewEgg, ) and borrow a USB optical drive to use a retail CD/DVD.
Download the Relevant ISO directly here (Content hosted on Digital River), and then use the Windows 7 USB/DVD download tool to install.


Home Premium SP1 (English, 64-bit)
Professional SP1 (English, 64-bit)
Ultimate SP1 (English, 64-bit)
Home Premium SP1 (English, 32-bit)
Professional SP1 (English, 32-bit)
Ultimate SP1 (English, 32-bit)



I'd wager that each of these methods would work for Windows 8, except the direct downloads, which link to Windows 7 only.