I rode Mavic GP4's for many years, and they seem to be pretty common. Most recently, before I gave up on tubulars 5 years ago, I tried the Vittoria Rally's, because they were cheap. Too many flats.

I met a guy in the bike shop in Middlebury, though, who rode thousands of miles a year around the Adirondacks and VT on tubulars. He swore by Continentals.