Lomotil is a “Schedule V” drug, with extremely low abuse potential (the lowest of the controlled substances). You do have to show an ID to get it, and you do need a prescription, but it’s just not a big deal. Anyone who takes it in quantity to get high is even more foolish than someone who abuses loperamide, because In addition to the constipation the atropine would make them miserable. Most doctors are perfectly comfortable with it.
”Fiber pills” probably means psyllium capsules, or maybe something like Fibercon. I’ve had great luck with psyllium, and take it every day, but the capsules didn’t work as well for me as the powder.