You will be hard pressed to find a junkyard 20R head, simply due to the age. Check with local parts stores for the availability of a remanufactured head. If they don't have a listing hunt the local Cragslist or post a wanted ad.
Another option if you can't score a good 20R head is
http://www.engnbldr.com/head_shop.htm they offer a pro topline made performance 22R head with a thicker casting, custom ports and for just over $500.00 it is ready to bolt on with the camshaft of your choice.