Almost all fibre comes from the main ingredients. That is from the oats and soy.
We use xantham gum to make the shakes mix well, but you could use much more of it as a fibre source.
In an earlier version we used to add inulin, but our shakes contain enough (the recommended amount) of fibre already. And compared to the average European diet, it's about 2x as much.
Feel free to experiment, but we think there is already enough in there