I agree with Robert Hall's response. Ammonium Thiosulfate in "powder" form is expensive and very, very difficult to store. I have some. IIRC, it's hugely hygroscopic. You might be best served looking for sources of color fixer, or as Robert said, possibly agricultural. It's used in certain farming practices as a nitrogen source. Hydroponics, I think.

I did pursue the ag channel a few years ago and found that I was better off buying it as a 60% solution. USA.