Hard to find parts!

I have a 1981 Isuzu Imark with a 1.8 liter diesel, and a 5 speed manual transmission. It has 290,000 miles, and still runs great! (Not to mention, 44 mpg!)

I'm doing a clutch job, and I have been able to find every part I need EXCEPT for the rubber gasket/shield that seals the hole where the clutch release lever comes out (on the left side of the trans--the one the clutch cable attaches to).

I've checked the dealer, parts stores and the internet, and can't find it.

I'm wondering if the same type part from another, newer car might fit. If so, how do I do the cross-reference? Or, is there a place that might make me this piece? I have already designed a work around using an inner tube, but I would rather use a real part if I can.

Any ideas?