Finding old cork gaskets is an endeavor you likely wont win. As Ed and Puffie40 have said, manila folder and/or RTV gasket maker are your friend. Be sure to check as some axle diff cover bolt holes are through holes and will need the threads sealed up with RTV or they will slowly weep.
When you assemble, coat both mating surfaces with RTV and mount with the bolts just beyond finger tight. Wait 2 hours and then tighten down to correct spec (wont be much).
