GG crackers. They are very sturdy, somewhat bland (not salty) crackers sold at Whole Foods, but cheaper on Amazon.com if you are in the U.S. and can use the Amazon grocery department. You can order them online from the website (the product is Norwegian and there's a U.S. distributor).
These things, with some tuna salad, or peanut butter, or spread with salsa and cream cheese (the rye taste of the cracker doesn't interfere): a small cracker has 12 calories and 5 grams of fiber. Amazing - not that they have so much bran (they are mainly bran) but that they are so mild in taste and are so sturdy.
They are extremely filling - be forewarned. They are expensive, but cheaper on Amazon, I think. I haven't ordered from other sources, so I don't know about that.
Lentils are my favorite source of fiber, and I add black beans to a pasta salad, just to pump up the fiber. I watch fiber in my food log and most days, I have around 40 grams! Protein is low in fiber so I try to get enough protein but not at the expense of fiber. The more vegetables and whole grains, the more likely you will be getting fiber in your diet. However, a salad is just not enough (lettuce is not high on the list of fibrous foods). A cup of shredded Romaine lettuce has 1 gram of fiber. But - it's very low in calories, too.