Sodium thiosulfate is a standard chemical for dechlorination of water.  It will not work on the ammonia half of chloramine, however.  It is available premixed, at your local pet shop, distributed by many manufacturers.  Or it can be bought in crystal form in 2 to 50 pound lots and mixed yourself.  Mixing it to a 13% solution will allow you to use 2 drops per gallon for standard dechlorination.  You can make this solution by adding 500 grams to a gallon of water which will be enough to treat 37,850 gallons, or 130 grams in a liter of water which will be enough to treat about 10, 000 gallons of water.  This is a standard dosing and depends on the amount of chlorine in your water supply.  When the chlorine levels are higher you may require a little more.  A simple pool chlorine test kit can be helpful if in doubt.