You can buy a new one
from Rudy at D&R Marine, but its expensive; I just checked on his website, and he is looking for $715 for a rudder with pintles and a tiller. Its hard to tell from your original post how you lost the rudder, but it seems unlikely that it just floated away. I've been in some pretty rough water with my Oday 23, and the rudder has never just "floated" out of the gudgeons. The previous poster has suggested that the rudder can float out under certain circumstances, but your original post made it seem like you put the boat away one day at the dock or mooring, came back later and the rudder was gone. It seems unlikely that the conditions got so rough in your anchorage (marina?) that the rudder was dislodged. Unless there is more to this story, I suspect foul play. Good luck looking for the thing; in addition to checking the shoreline/marinas in the area, I would also stop by any nearby boatyards and let them know what you are looking for. Perhaps someone has come by recently to see if they would be interested in buying a slightly used rudder.