You can go to a 36t ring on a 110 bcd crank, I believe. Otherwise, in addition to 50.4bcd cranks (TA, Stronglight, Nervar, the rare Shimano PX, plus new ones from V-O), you could opt for an 86 bcd crank and go as small as 28t, I believe. Those were made by Stronglight, SR, and probably others.
Another option is to attach BikinGreen 46/30t rings to a 110 bcd crank:
