Until Steinberg answers, I believe they won’t sell you any version but their newest versions…why would they? They’d just assume sell you a current one at full price.
If you want version 5 or 6, you’ll probably have to look on line, and then be patient. Ebay, Amazon etc. I know ebay was selling new versions of 6 last year, and there was an upgrade path to 7 from there. I almost took the bait, but decided I didn’t want to upgrade literally everything else…Cubase, my audio interface, my entire computer, operating system, plug ins, VSTi’s, etc, etc, etc.
As far as I’ve read, to use Cubase 7, you’ll need Win 7 to run it on. Maybe someone here has their old Cubase 5 they want to sell, but have no idea of the cost of transferring a license. With all the older Cubase versions out there, there must be some available…you’ve just got to find someone selling it. You won’t likely find it through steinberg.