There is a specific "refurbished" version - the COA sticker reads W7 for refurbished PC's. My local repair shop use it and I have seen it on a couple of PCs I have set up for people.
I don't know the limitations of a refurbish version or pre requirements before it will install but once installed it is no different.
Its even becoming more difficult to find any COA references nowadays, unless its just me having problems. Only recently I purchased a new Levono laptop, and the Windows 8 information sticker was attached to the battery.
I still say that not supplying a disk with new computer's nowadays is not a wise move. Surely the manufacturer's cannot be making a great saving in not supplying?.
But back to the original subject, I have purchased the varies forms of genuine Windows O/S disks from a number of sources, including Ebuyer, Ebay. Amazon and other well known outlets, usually at discounted rates. But you do need to shop around.