Sodium sulfite is probably the most-stocked photography chemical, and it should be available off the shelf in any halfway decent photo shop (even the ones that have mostly "gone" digital), or you can get it through other sources, such as Photographer's Formulary. You can order online from the Formulary.
You could also probably use Hypo Clearing Agent.
I envy you for still having some 665. I didn't care for using 55 in a 4x5 camera all that much, but I loved using 665 in a Mamiya Universal or RB. I hope it still works!