Some time ago, I bought a used Giant full suspension MTB. The guy cut the original seatpost too short for me, so I needed a new one. The post was some weird size, either 31 or 31.1 mm. Closest I could find was a 30.9. (Isn't that the standard?) Needless to say this kept on sliding down as I rode. I stuck a small strip of metal between the post and frame -- yeah, I know you're not supposed to do that! -- but it still slowly creeps down (about 5 mm every couple hours). I checked eBay and Giant's web site, to no avail. Anyone know if I can find one somewhere? Or anyway to fix it? (Its amazing how hard it is to slide the post up and down with that homemade "shim" in it, and how hard I tighten up the clamp -- surprised it hasn't broke yet -- yet it STILL slides down!)
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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.