Actually, it is not so much "where" you bought, but "what" you bought. You need to check your license and determine the version of Windows 7 you have. Some "retail" packages contain disks for both 32-bit and 64-bit (but you can legally only install one). If you have one of those, then you can upgrade for free. But most OEM (system builder) licenses and upgrade licenses only come with a 32 or a 64-bit version disk. And Windows 7 "Starter" version (often used with netbooks) only comes in 32-bit.
In any event, I agree with the advice given. Give Microsoft a call and they will help you determine what you have, and your options. But first, before moving to 64-bit, check your hardware makers' websites to make sure they have 64-bit drivers for all your hardware.