Hi, had a good luck but it's not been too easy. Most of my oversized posts are in the 'standard' oversized size, and the one you're after is a good deal less common. Anyway, the closest I came was 30.8mm but I believe this size just caters to a particular brand (Giant?) of bike, and yours isn't this brand. I think yours has a fairly common oversized size but it seems I've run out of these, and I felt confident yesterday I'd have it seeing as I though I had most of the common sizes, but not in this case it seems. I do have a few other places I can have a 'last try' search, but this could be a week from now. It's a pity though, because I turned up a good few of the seat clamps, but obviously these aren't too much help for you without a post to match it with.