You need a thiosulphate.
Either a Sodium Thiosulphate which is a powder/crystal and generally slower acting whereas the ammonium thiosulphate is usually a liquid.
If you can find sodium thiosulphate (usually easier to find than ammonium thiosulphate) you will need to adjust the amount in your formula.
If you can tell us where you are located we could proceed to suggest some possibilities.
Sodium thiosulphate is used in swimming pools and can sometimes be sourced from a pool supply place.