Classic Parts, page 115 in catalog, $3.29 each. I would also get one for your differential if it uses a hose and vent, and maybe an extra one in case one pops out of a dried out hose in the future. Vintage Parts might have them also, but I don't know the GM part number, which is necessary if you shop there.
http://www.classicparts.com/1947-91-...ctinfo/93-245/