You buy snowboard boots a half a size smaller than what you normally run.
Just pulled the trigger on a pair of 32 TM-3s in an 8.5. I’ve only tried these on in a 9.5, and normally run a size 9, hopefully footbeds and some experimental bootfitting do the trick. As much as I enjoy the masochistic experience my foam injected Salomon Synapses offer I’m looking forward to being back in a regular, albeit high-end, boot liner.
Also got a fleece too, because you can never have too many layering pieces.