Disclaimer: I am just a shade tree mechanic (of that), so anything I say could well be completely wrong.
A guy I was talking to the other day about some lockout hubs said that the axle snap ring was only 100% essential on the very very old models, and that even my '66 would probably be ok without one. But I am going to try to find one anyway, as I have an uneducated theory/supposition that not having it is what broke my hub in the first place.
If you take the snap ring off the other side in for measurement, I would think that about any decent auto or farm equipment parts store ought to be able to match you up one that will work ok.