i have used the link below to buy used sails. you may find something from another boat close enough to use if they don't have an oday 20 sail.
they have a listing of demensions for the oday 20 and i believe they sell new sails also. i purchased a thistle jib which i use on my oday 20 when windy. this jib, along with reefing the main on the roller boom, allows me to sail in windy conditions without worry.
www.baconsails.com