John - It's known in the Chevy Master Parts Books as "Spring, lock" or 'Retainer, hairpin" depending on which part of the vehicle it is being used on.
GM# 355936, Spring, lock - Parts Group 0.837 as in your application, or Retainer (hairpin - 1-3/4 long), Parts Group 0.376 as the retainer at the ends of the 216/235/261 rocker valve shafts.
No one appears to be offering any of these on-line, but you could snag a few from a derelict engine easy enough.
You could also experiment and try to find an E-clip with the same shaft diameter to use in place if all else fails.
