buy some. LFS's often have caulerpa for sale, but they may have other species.
you can also go to asian markets and buy nori (a type of seaweed/kelp) that isn't alive but is fresher and not all dried out (so more nutrious).
Former advisor and planted tank geek...life's moved on though.
I think the last time I was at the fish store they had these green leaved plaints that they said was good for the tank, also help reduce nitrates ,I put some in my tank that the tang I hade ate them clean
if you have algae grazers in your tank, it will be quite hard to grow it IN THE TANK,
as they will eat it before it can grow. If you have a refugium, it could grow there and then you harvest it and feed the fish.