Ascorbic acid is one option. There are quite a few experiments (and studies) out there involving the use of this form of vitamin C to neutralize chlorine in the realm of water treatment. Additionally, ascorbic acid can be purchased online or in certain health stores in capsule, crystalline, or other forms.
If you are interested in making your own mixture to remove chlorine from your hair, do read more about ascorbic acid as an option.
VITAMIN C DECHLORINATION IN WATER SYSTEMS
VITAMIN C (ASCORBIC ACID)
ASCORBIC ACID REDUCTION OF RESIDUAL ACTIVE CHLORINE IN WATER (2000)