Unicycles are for clowns
Finding that a 26.4 seatpost is kind of hard to come by these days. Anyone one heard of Spectre? I found a seatpost that is made by them in 26.4, from early 1990's, but I don't seem to recall ever hearing about this company, my guess is a small machine shop?
You could use a 25 or 25.4 with a shim, or look for other ones. I did a quick search and came up with this:
Thudbuster (It says you can order it here, but I check their site and m not sure if this one is right).
Weird ebay link
Other normal ones pop up on ebay. I just searched completed items and found some. It seems more like a BMX size so you might just want to look for bmx posts. I think the Kalloy is best pick, but then again, I'd still wait for an answer on the spectre post.