www.sheldonbrown.com
Do a search on seatposts. While you are there, do a search on fixed gear and track bikes, and see what he said about converting track bikes for street use (just so you are aware of the issues).
I think you will find that seatposts come in diameter increments of 0.2mm. Some sizes, I think, aren't that easy to locate. Yours might be in that category. eBay is one place to start. You will need to look at length, too. Google searching might also help with "26.8mm seat post" as the key phrase. Note also I-beam; although I have no experience with them, I-beam seatpost clamps need I-beam-specific saddle rails. And, seatpost clamps vary in their usefulness. Again,
www.sheldonbrown.com discusses the issues.
Finally, the SS/FG forum is a goldmine and cesspool of information all at once. It might be useful posting something over there. roadfix is one whose opinions I would value.