You can get 27mm from FMB, Dugast and Challenge. Larger tubulars can be had, but they are for cross bikes, have knobby tread and will have terrible rolling resistance. Tufo and Dugast do make a 32mm cross tire with a diamond tread (for racing on grass). Not as bad as knobbies but definately not fast like a road tread.
I have a pair of Vittoria butyl diamond tread cross tubulars, 32 mm, lightly used and never flatted, for sale. PM me, anyone who wants them for an experiment.