Make your own, take a block of sphagnum moss peat, shred, put in an old fashioned (not plastic!) sack, and lob it into the sea so itis covered,then uncovered by the tides, and leave to soak for a couple of weeks. Squeeze out excess seawater , and there's your sea peat.
Just make sure that the peat you use doesn't have any unwanted additives like fertilisers, NPK, etc.
Good luck with it, it worked for me!