Climax metal products is the actual manufacturer of the part.
2ISCC-050-037-S stainless steel http://catalog.climaxmetal.com/item/...iscc-050-037-s
2ISCC-050-037 black oxide http://catalog.climaxmetal.com/item/.../2iscc-050-037
Amazon seems to have a few, ranging from mid 30s for the black oxide to $61 for the stainless. Ebay seems to have them too, but not sure if they are knock-offs or actual brand-name parts, but at substantially lower prices it appears.
Note, some of the sites only list one of the bore diameters, which is why I included the links to the actual manufacturer parts that shows the XXXXX-050-037 parts are 1/2" and 3/8" bores.