Stranglekelp is required for many different potions and sells for a good price on any server. It isn't as hard to find as many people believe. There is tons of Stranglekelp all over the Zoram Strand in Ashenvale.
You can swim the route from southern to northern Zoram Strand picking up Stranglekelp as you go. When you reach the northern side most of the Stranglekelp behind you will have respawned enabling you do to the same thing over and over again. You should be able to gather a couple stacks of Stranglekelp per hour.
The mobs on this area are pretty weak (18-21) and relatively far apart from each other so you don't need to be on a very high level to do this. Gathering Stranglekelp requires Herbalism 85+.