I need to buy/find product key for activation of my window vista... they tell me

I don't think Microsoft is issuing any more Product Keys for Vista now that it's been discontinued, but your PC may not be compatible with Windows 8 either if the manufacturer has no Windows 8 drivers for it, so that may not be an option.

You could always consider a Linux distro like Ubuntu. It's free, no Product Key and no activation required: http://www.ubuntu.com/ubuntu

If the machine you need to activate Vista on, came with Vista, the product will be on a sticker either at the back (if it's a desktop) or at the bottom (if it's a laptop).

Or you could pick up an old Vista retail version on ebay or some other online site. Why Vista at this late date?

By "this late date" do you mean the thread started in February or Vista has been declared useless since it came out in 2006?