My favorite and most effective supplement is triphalla from Now. They changed their formula so i have to smash them now and drink them, but let me tell you, I've got amazing monster poops thanks to triphalla. (I take 12 per day so you might have to take more that you would expect to see a difference.). Also, magnesium citrate helps to bring water into the colon. I also eat ground flax seeds and flax seed crackers and that helps to add bulk and seems easy to tollerate. I special order my flax seed crackers from a company called Go Raw.
Good luck! It's really important to get yourself regular, otherwise you are reabsorbing toxins back through the lining of your colon. You can't get well that way.