Are you building from scratch? If so, Dexter Torflex axle is my recommendation. Mine is over 11 years old now and still smooth as can be. Independent torsion arms, completely customizable.
Here's a shot when it was new back in '95:
It's been revised since then, moved the skis forward, new bow stops (get Stoltz polyurethane, there's a thread on where to get them now), ditched the aluminum box for a plastic one that doesn't RATTLE or cover everything with aluminum powder..
The main frame is 2x3 galvanized H-beam, fully welded, all hardware is stainless, including the skis/tiedowns/handle/fender mounts.