try these

1. psyllium husk seed fiber (metamucil) - you can buy powder from walmart or walgreen or GNC. This fiber is not considered laxative, so you don't need to worry about side effect;

2. flax seeds + apple + milk or soy milk - this one you need to use kitchen blender to break the seeds. But it is a lot more nutritious than the above fiber alone drink. The flax seed fiber is one of the best kind, same as apple fiber, retains moisture.

Either one shall solve your problem. Good luck