[DeIlliad]

Those foods specifically aren't cheap foods but foods high in soluble and insoluble fiber can be found very cheap. Whole grains are high in insoluble fiber while black beans and oats are high in soluble fiber.

[InSteady]

Most veggies are reasonably high in both. Go to a farmer's market if you can afford it. If not, go to a cheap (often ethnic) grocery store.