Consider USE, UK, they make 25.0 posts and shims, 0.2mm increments, to 27.2,
and a 27.2 post and shims to larger than that.
AlAn, frames used 25.0 posts shed the frames,kept the seatpost.
bought USE shim ,a 25>27, so I fit it onto my RB1.
Hurry, the aluminum ones are in the close out store
http://shop.use1.com/category-s/91.htm