Hi and welcome to FixYa, I am Kelly,
The part number for the 20W light bulb is 4612S16400. It has been discontinued and is no longer available no matter where I have checked. I started my search for your bulb 2 minutes after you posted your question. You may be able to carry the bulb into a LOCAL appliance parts supply store front and find a suitable alternative. Do not install anything larger than 30 W but you can use anything from 15 - 30W as long as the bulb will fit in the socket.
PAC Parts is the primary source for Sanyo parts and you can see the result I got there:
Repair Clinic Same result
Union Electronic Distributors
I wish I had better news but this result is what the results of an after market search have revealed.
If your good with appliance repairs here are some alternatives: (Only first 5 entries)
Thanks for choosing FixYa.