You can find the Psyllium, Stevia, Bentonite in any decent healthfood store yes. You may even be able to find Stevia in a decent grocery store.
As far as the 25 mile commute, I can relate. you'll need to be sure you do the salt water flush..plenty early. Occassionally, it takes me 1.5 hours to fully flush. But then again I'm a big guy and perhaps that matters.
As far as the Bentonite and Psyllium, I just experimented this evening and added the 1 TBSP into a 10 ouce glass. I'd say that it swells up so much that I think it would your dependability or timing your BM's, not make them less dependable.
Just remember one thing. There is an interesting nervous system signal within our bodies when we eat. It works like this, when we eat and the stomach senses that we have food. As the food passes into the duodenum (first part of the small intestines), a response urge to evacuate the rectum is sent. Causing us to visit the restroom.
So..not sure of your schedule, if you dont have enough time to take the salt flush, wait certain amount of time, take the B&P and then wait a bit before driving, I think you might be best to bring the B&P ingredients to work, add the water there, and deal with a single restroom trip once there.
Just my thoughts. And not any true experience, since I'm starting my B&P's tomorrow. Just A&P training.