Bud - Take the IHC seal p/n (357 037 R91) to NAPA. They can cross it to another mfr's equivalent and order it for you. (I believe Timken #450316 is one equivalent - there are several others).
Keep in mind the new seal will be only 1/2 the thickness of the original. Can either install two seals in the counterbore and fill the space between the 2 seals with grease **OR** install a single seal positioned so the lip rides on an unworn area of the bull pinion shaft.
I prefer the latter option.