Well, Jafo's post above suggests that you might still find a copy of Win7 to buy if you can finds a store that still has some in stock. Finding a store that has a copy and would sell it to you would be the hard part.
But what I was told is that it can't be, because when a seller gets a license from MS to sell the current OS, part of the contract is that they can no longer sell any older ones regardless if they have copies still in stock. If that's correct, then you'd not only have to find a store with Win7 in stock, but one that was willing to violate their license and risk somehow being caught and losing their license for the current version and all future versions. At least that's how it was explained to me by the shop that initially built my Win7 rig. My techie friend who rebuilt it for me (who I didn't know yet when it was built initially) didn't dispute that, and added the part about serial number linkage - which is part of MS's effort to force us to keep buying new confutors and/or Windows versions when we'd rather upgrade or rebuild.
If you could find a copy that was sold as just an OS in a box, meant to be installed on an existing confutor, not linked to an existing processor or motherboard, that would be great! But finding it might be a fruitless quest. If you do find one, I concur that you should install it immediately and get all the updates while they're still available, which they won't be after some time in January.