If you take the lid off the toilet there is a cartridge in the middle of the tank. if you take the handle mechanism off the toilet and get some plyers you can take the cartridge out of the tank by twisting it out. make sure the water is off and that you have pressed it down ( the part that stick out of the middle of the cartridge)to relieve the pressure. it should come right out and take it to a local plumbing supply shop and they should be able to get you a replacement part for it.
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If you take the lid off the toilet there is a cartridge in the middle of the tank. if you take the handle mechanism off the toilet and get some plyers you can take the cartridge out of the tank by twisting it out. make sure the water is off and that you have pressed it down ( the part that stick out of the middle of the cartridge)to relieve the pressure. it should come right out and take it to a local plumbing supply shop and they should be able to get you a replacement part for it.