Nitto makes (made?) 26.0 mm diameter Randonneur handlebars. Ask your shop to order them from Euro-Asia or some distributor like that. They have different models (Nitto, that is), with different reach, width and drop. Rivendell used to sell the model that made most sense - a long reach one.
Note that many Nitto bars are copies of French bars, but sized smaller (presumably for Japanese hands) - the opposite of what most of us Americans would like.
Of course, I agree with Sheldon - shims on bars are no problem. I once saw an Herse that was equipped NOS with Cinelli stem and Philippe 25.0 mm bars. Inside was a nice shim, appears to have been a common combination.
Jan Heine, Seattle