Why not just increase your intake of fibers and water. Lack of these are the most common reason people get constipated. Rice bran and oat bran are excellent choices. They are cheap, effective and very nutritious.
Low thyroid is also a common cause. Checking basal body temperature is the easiest and most accurate way of determining thyroid function.
Things I recommend avoiding are:
Stimulant laxatives. These include senna, cascara sagrada, cape aloe (aloe ferox) and rhubarb root. The high anthraquinone content of these hers can weaken the intestinal muscles leading to a laxative dependence.
Magnesium oxide/hydroxide. This is normally sold under the name Milk of Magnesia. The reason this form of magnesium is used as a laxative is because hydroxides are caustic. The magnesium hydroxide burns the intestinal wall leading to a release of fluid in to the intestines and increased peristalsis. Although repeatedly burning the intestinal wall also increases the risk of intestinal cancer.