Anyone make a 31 or 31.1 mm seatpost?
According to info from Sheldon Brown, Your Giant post is most likely to be 30.8 as this seems to be a common size on with that brand - however you don't give the age and model of the bike, so it could be Giant changed to another size. The conspiracy theorist in me thinks its so they sell a whole new bike when to can't find a matching seat post. 

I have also had this problem (With another brand), and beyond trawling ebay and craigs list in the vain hope sellers will put the size on the listing and its accurate, your best bet is to try your luck with your corner LBS (Not the chain-shop who only has this years stock and is shiny and flash) - good chance they have one in back somewhere. Shops that have been around for a while tend to keep boxes of odd ball sized parts such as these, (rather than throw them away) as it's good for business being the "The only guy in town who...." kind of place.