seatpost sizes

Hey fellows, i’ve just got a new frame, just wondering whats the deal with seat post sizes?
the frame specs say its a 31.4 but i cant seem to find any seat post with this measurement.
does this mean ill just need to use a shim??

The main issue is getting a nice seat post in that size. But they certainly are available:

or even

I know how you feel - I’ve got an old bike that takes a 26.4mm post… Kind of hard to find other than crappy kalloy ones. Ebay ended up working for me in this case - in fact there’s a fairly cool lookin Selcof and two FSA 31.4 MTB posts on the 'bay right now - search for ‘31.4 seatpost’

cheers dude