Use a Stegosaurus and harvest berrys to get the seeds

To get longrass you need to grow it yourself, in a medium or large crop plot (either way they grow at the same speed), you need to water your plants with an irrigation system or cantines and fertilize them with poop or fertilizer (made in a compost bin with thatch) and over some time you can find longrass appearing in the crop plots, i reccomend leaving them in the crop plot until you need them cause they expire very fast outside the crop plot

P.S. on some maps you can find longrass and other plants growing in the floor in abandoned villages!

Use dinosaurs to attack bushes, then check their inventories. They'll have a bunch of seeds and sometimes have longrass seeds.

Just get your dino such as bronto stego parasaur (ect) and wack the bushes then you will get seeds ang plant them in a medium or large crop container (I'd recommend to keep it in there until you need it because it spoils very fast outside of it)

If u wanna get some of these I would say take your anky on an argue up there and harvest some bushes or the rare flowers too get some.