Good place to buy Windows for Parallels/VMware

I have been looking for a good place to buy or download Windows for use with Parallels/VMware on my new iMac (when it arrives). I have looked at most of the normal online resellers but mostly find OEM versions on disc. I suppose I could buy one of those and create an iso file from it, but was wondering if buying an OEM version was such a good idea considering that I might want to end up buying a MBP as well and could not transfer the Windows from the iMac to the MBP if it was OEM. My assumption is that I could not use any version of Windows on both machines, even if not at the same time. Or is that wrong?

Where have those people who have installed Parallels or VMware for a Windows installation gotten their Windows version? Is there a download version? The iMac will not have a dvd drive.