Why do you want a mangrove? If it's mostly for the look, that's OK. If it's for nitrate export, they don't grow fast enough.
Mangrove propagules fall off the tree and survive for weeks and some even for 2 or 3 months if they are in the water. We pick them up off the beach, stick them ina glass of water (salt or fresh) and 80% of them put out roots ans start to grow. I've had a few in a shallow tank I had (4'x2'x14") and they do OK. However, they tend to outgrow your light in height, they expire salt out through the leaves which is normally washed off with the rain (you'll want to wipe them off or they die) and the roots can spread pretty far in the sand.
I wouldn't pay much for one that is already grown. It would be hard to get the roots buried in the sand and it will just reach the height of your light that much faster. A propagule can be bought and shipped VERY easily and they are pretty cheap.
Heck, I was just in Moab a couple of months ago and had a great time. If you can't find propagules for sale fairly cheap (under $10 each) let me know and I'll mail you 4 or 5 when they are in season.