I use linseed (also known as flax seed) or chia seeds, or any of the mixed seed mixes you can get now. Milled (crushed) works fine for me as whole ones get stuck in my teeth and annoys me and my dentist.
I started with up to 3 tablespoons a day mixed into yoghurt but am now down to a tablespoon only every day or so.
I was recommended these by gut doctor as I have diverticulitis which can get inflamed . He told me never to get constipated.
These seeds I find are magic, they seem to loosen up the firm, and firm up the loose. Easy to adjust amounts to suit you.
Now on low carb I also use them to thicken yoghurt as a pudding, or to make 90 sec mug bread, or in grilled cheese to make crackers.
Lidl sell good mixed packs at reasonable prices in the cereal aisle, or I get larger packs from health food store.
Keep opened pack in fridge.
I've recommend them to many friends with digestion problems, and all have said they work