There are bran cereals that have no to almost no added sugar, are fairly low in calories per serving (like 80 calories per serving), and are almost just pure bran manipulated into pellet or noodle form.
Also, I like eating some oatmeal with a scoop of protein powder in it. If I make the oatmeal a little watery I can mix the protein in (I like chocolate flavor in oatmeal the best) and not need any milk. Oatmeal has more carbs than just pure fiber, but oh well, I'm definitely not religiously against any carbs - just keeping the carbs down to a dull roar.
One thing I did in the past when I was being more nazilike in my carb count, and wanted a lot of daily fiber, is to take a large spoonful of sugar-free metamucil in the morning with a huge glass of water. That's really just pure psyllium husk fiber, and will definitely, uh, keep things moving down there.
But again, for me personally it's easier to eat very low carbs if I'm not also trying to ride long and hard. Long and slow is doable, and short and hard is doable, but long and hard, for me, simply requires some carbs. So be it.