The first thing I would do is get an accurate measurement of the post that does fit. If it is close to the 26.4, like say 26.2 you might consider running an adjustable reamer in the tube. Many production bikes simply are not round, sometimes due to distortion from brazing, or from over-zealous tightening. An alternative is using the Stein seat post sizing rods that are stepped in .2 mm increments.
Mark Hoffman
New Britain, CT
USA