As you should know there are two versions of windows, one for pre built systems pre installed with the system.
And called Oem versions, often sold with the pre built machine by Dell, ect where the OS is often locked to the system specification and will only activate windows os if the system specs and hardware of it match.
The retail version of any windows version does not have the Oem lock down and activation.
But it does and can come with a limited amount of times it can be activated. and is One os per machine.
Meaning you cannot install the os on two machines running the same os with the active licence key at the same time.
If you want a retail version then any good local computer shop will stock what version including windows 7 and the bit version you need. Or such a place as Pc world.
Just do a search for retail version of windows 7 and the bit version you want 32bit or 64 bit.
If your system has more than four gigabytes of ram, buy the 64 bit versions of the retail verson of windows 7.