Psyllium consists of the indigestible seed husks of a plant native to India and Pakistan. The seed husks are a good source of soluble fiber, and thus act as a natural laxative. As such, they are also a popular ingredient in colon cleansing and detox regimens, and are also used as a binder in gluten-free baking. However, some people cannot tolerate psyllium either due to taste and texture, or because of side effects such as gas and bloating.
Flaxseed is widely available, and is high in soluble fiber and omega-3 fatty acids. The soluble fiber content may make it an effective natural laxative, but MedlinePlus notes that no studies have actually proven this assumption. Flaxseed has a pleasant, nutty taste.
Oat bran is another food high in soluble fiber, and bran powder is used as an ingredient in over-the-counter laxative formulas. It can sometimes cause gas and bloating, so add it to your diet gradually to let your body get used to it.
Pectin is a soluble fiber found in high amounts in certain fruit, especially apples. Apple pectin can be found in capsule form as a dietary supplement marketed for lowering cholesterol and increasing dietary fiber; it also acts as a natural laxative. Apple pectin is another popular component of detox and cleansing regimens. If you're just after the daily dietary fiber content, however, there's no need for expensive capsules -- an apple a day will suffice.
Dried fruit, especially prunes and figs, are high in fiber and have been used as a natural laxative for centuries. You can eat them as-is for a snack, add them to your hot or cold cereal, or stew them and eat as a breakfast side dish or over whole-wheat pancakes.