Ammonium Thiosulphate can be created in solution by mixing Sodium Thiosulphate (plain Hypo) with Ammonium Chloride. Ammonium Chloride is commonly used as a fire retardent treatment.(posted 8523 days ago)
The ratio is 40 parts by weight Ammonium Chloride to 60 parts Hypo, if my memory is correct. This reacts to form Ammonium Thiosulphate and common salt. The Sodium Chloride takes no part in the fixing action.