Legumes are loaded with vitamin B9, I wholeheartedly recommend them.
By dry weight, chickpeas and mung beans have the highest content, about 600mcg per 100g, navy beans 400mcg, green lentils 480mcg and red lentils 200mcg. You can also buy chickpea flour that's 440mcg and make flatbreads or banana bread with it. I did a whole year of high legumes diet so I managed to get 800mcg of vitamin B9 from food, sometimes even more and it was delicious. Falafel is awesome.
As long as legumes are soaked, rinsed and cooked long enough, I found they're easy to digest. Red split lentils in my experience are good to start with. Regular veggies like cabbage also are a good source, but it is usually below 80mcg. You can eat liver which is very rich in vitamin B9, 590mcg for chicken liver and duck liver has even more, they also have a lot of B12. Many people can't tolerate different B9 supplements, so there's always food.