I believe the retail versions have separate 32bit and 64bit dvd's in them. The OEM version has 4 options. 8.1 32bit, 8.1 pro 32bit, 8.1 64bit and 8.1 pro 64bit. And if you buy digitally you get what ever version you paid for.
TBH I don't know why anyone would want a 32bit version.