I want to harvest it. Is it rare?

I haven't found any harvest points yet no. I had to find it as random drops, then plant it in a settlement to start getting it.

Some settlements have it not sure which ones tho

Because I've done quite a few Minutemen missions I've seen it in another settlement and harvested it. I've also found it in an occasional container and soemtimes on general merchants like that lady with her junkie son at that diner SW of Starlight.

I actually find it on new settlers I get.

Buy it from merchants, plant it and then spread it. Making Mirelurk Cakes, huh? lol

Have found it on bodies occasionally, and for sale from merchants sometimes. All you need is one, once you plant it just come back and harvest it, and replant etc.