See page 2.3 for the valve part numbers to use for acquiring new parts for your 108. http://stevespages.com/pdf/crosman_factory_service_manual_all.pdf
These larger "valve nuts" (105-8) are superior to the original small valve nuts seen in your 1949 gun.
Most shops that carry these parts use the 121-7 nomenclature for the valve washers. Some shops have both a hard and a soft version. Hard is for higher velocity performance and the soft are better for holding air.