Buffalo Arms has it in stock, only game in town……depends upon how badly you want it:

https://www.buffaloarms.com/500-nitro-3-1-4-cases-by-bert-ber500314ne.html

This is, of course, your option if you want properly head stamped brass.
.500/.450, .465, and .470 (all 3 1/4") can easily be necked up to make usable .500 3 1/4"

- Mike