All the major RO makers use filmtec membranes. The only thing you need to know is the diameter and length, and there are not a lot of choices. Even from outside the housing you'll know which one is right.
Then just chose a Filmtec desalination membrane that fits. WAY, WAY, WAY, cheaper than buying
from the supplier of the ro system.
Note: There are lots of types of RO Membranes, you need the ones made for seawater desalination. They are clearly labeled as such.