very hard to find the seal nowadays. there might still be some out there (heavy truck parts stores) but you'll have to search. about $6 a piece.
this was a identical leak to what i experienced. while unsuccessful to locate a seal, i tightened the retaining bolts (totally 4 of them) about 1/4 turn, accessed from the top of engine, it stopped leaking ever since. prior to my fix, i talked with a diesel mechanic, he quoted me $500 to replace it. He said no other ways around