You can buy either wheat or oat bran at some health food shops; flax seeds are good, as already stated. Rhubarb is high fibre and can be sweetened with (eg) Splenda in powder form. Drink plenty of plain water. If you are a low salt user, like me, extra salt helps too.
I too have problems with constipation and always have had on low-carb. All the above help; if all else fails I resort to Andrews' Liver Salts, but that's not often.
Another thing that helps is allowing enough time for a bowel movement. Don't rush things. Take the paper in with you - and be prepared to be shouted at by the rest of the family! :lol:
Viv 8)