The mfgs buy axels, and you can too. I'd get on e-trailer or the internet and they'll have axels that fit your trailer.
There are some standards of measurement that you'll need to follow to ensure you get the exact ones you'll need. Doing it yourself is a pain because of all the rust. Not bad if you have cutting, grinding, and sawing tools. You'll save some money this way.
It's hard to tell from the pictures, but if you change, I'd go with torsion springs over leaf springs for saltwater. They will both corrode, but torsions seem to last longer for me. Good luck