An increase in fiber helps peristalsis work better and will give you more complete BMs.
Best sources in my experience are plenty of fruit and veggies, but never bran.
Oats can be helpful, but some people with IBS are gluten intolerant and have to avoid all cereals except rice.
If you want to take a fiber supplement, psyllium husks are helpful, buy them from your health food store, or pharmacy.
Everyone is different, you have to find out what works for you. Good Luck!
Please try psyllium seed husks. I`m using a product called Fibre Force that contains mostly psyllium but a couple of other ingredients useful for bowel evacuation. So far it has worked wonders for me. I tried various other meds and supplements to no avail before I discovered Psyllium.