A low flow toilet can save many gallons of water per year, both preserving the environment and lowering your water bill.You can also try putting a bottle of water in the tank of your current toilet. The space the bottle takes up will save that much water each flush, yet allow the toilet to keep functioning properly. (Dont use a brick for this purpose, because bricks can disintegrate.)
Try lowering the water level in your toilet tank. Many toilets have an adjustment to lower the float controlling the fill valve.