What kind of bike is it for? What type of seat attachment is it? Might be easier to buy a straight piece of plumbing pipe and fit a collar on it. Most lightweight seatposts at that diameter won't be strong enough to go that long, they will bend. Frame too small?
It's a mamachari - I love the comfort for the about town errands; just need to get more extension for my legs. Ideal size is 25.4mm x 600mm+, enabling as much post as possible in the frame for safety. A few years back I canabalized an old folding bike - the seat post was perfect for the mamachari I had at the time. Now I'm back in Tokyo looking for an extra-long folding bike seat post all the while wishing I would have kept what I had. Regarding 'plumbing pipe', reworking an end so as to enable a fit for a collar seems to be an obstacle. If you have any ideas, please send my way. Thanks!
For the cost,cheaper than a specialty seatpost it's probably easier to buy a used 28 inch mamachari. They typically have a larger seat diameter which will support the longer length and they are also bigger all over, much more comfortable. TBH I think your search is likely to be futile. 600mm pipe at 1 inch diameter will bend as it's not thick enough to support the leverage.
Not sure what you think is reasonable but if you were buying new you could get one in two days. I had an aluminum Bridgestone 3 or 5 speed chari that was good for around 28kph cruising. They say it's near impossible cause they want to sell you what they have in stock. http://page13.auctions.yahoo.co.jp/jp/auction/r146919077 New ones on Rakuten are another 5,000 yen or so.....