I use bottled prune juice. There is nothing on the label about any additives. While I imagine it would be great to make your own juice from plums you've dried yourself, I go with what is available and affordable when I need it. Can you buy the prunes themselves at the grocery store and make your own juice if the juice sold there is full of preservatives ? This is probably the best natural laxative around.
I did not find any great benefit by adding flax seeds them selves to the psyllium. I have used flax seeds both whole and ground for cleansing and taken the oil capsules with psyllium shakes. I have a hard time finding really fresh flax seed oil so I tend to grind the seeds myself and eat them as soon as possible after grinding.