I'm after the same thing in 27mm.
Believe it or not, the Serotta forum
http://serotta.com/forum is a great place for info on tubs for road use
Peter White Cycles is also helpful
http://www.peterwhitecycles.com/tubulars.aspAs far as I know, these are your choices. I don't know enough to recommend any of them:
- Veloflex Roubaix, 24 mm. These have lots of fans.
- The Vittoria Paves mentioned by Gus. They also have lots of fans, but the green is truly bilious. They made some limited edition ones in black a few years ago - maybe you can find some old stock.
- Dugasts, which cost tons. (I'm getting some in 27 mm with a kevlar belt - special order, taken me a year to get 'em.) Apparently I might regret this because since their takeover Dugast have been Not What They Once Were.
- The new Continental Grand Prix. They do them in 25 mm but I haven't seen any reports of them
- FMB
http://www.fm-boyaux.fr/piste.htm They get very good reports as 'the new Dugast' and I wouldn't be surprised if they would be open to doing you a special order in 25 mm with a belt? I dunno - ask! I suspect you can't afford to wait forever though.
Have you done big distances on tubs before? What have your experiences been?