Correct me if I’m wrong, but I don’t believe he recommends taking fiber supplements. My take was that he suggests a diet rich in fruits and vegetables (including legumes). This would cover all your bases: soluble fiber, insoluble fiber, prebiotics (which as you may know is basically fiber), and vitamins, as well as the secret weapon of vegetable consumption: your have less time or room to eat other less healthy foods. If you can get the fruits and vegetables local and organic, then with only a very light washing with water you can get all that added soil and bacteria into your gut to helps things out too. Whole grains are another huge source of fiber. The healthiest and perhaps most ethical meal out there is a vegetable, a bean, and a whole grain (aka fiber, fiber, fiber).
If I had to recommend a fiber supplement–I would suggest chia seeds. They have a whopping 10g of fiber per oz (plus almost 5g of protein, and almost as much omega-3’s as salmon) and can be sprinkled on almost any meal throughout the day (salads, oatmeal, etc.). I like to keep a jar just sitting on my countertop. Added bonus: it’s super cheap.
Be a little careful with big doses of fiber, it could cause gas and bloating, as can overeating carbs (because your intestines can only absorb a set amount of carbs per hour and all the leftover gets fed to the gut bacteria and can cause GI upset too). Fruits, unfortunately, can also be complicated from a GI standpoint. Check out the FODMAPS website at Stanford Health, or elsewhere online–it can help you pinpoint whats giving you that bloated feeling, if it occurs–hint: it’s almost never gluten.