Which hydraulic throwout bearing?

Seems to me somebody needs to hit the junk yards, parts stores, and catalogs. What I mean is, SURELY there is an OEM than can be used, adapted. Just as a 10 second off the top example, here's some sort of GM replacement for about a hundred. No idea what the ID might be.....

http://www.speedwaymotors.com/GM-Stock-Clutch-Hydraulic-Throwout-Bearing,1945.html