Looking for sailboat trailer axles?

Ok, I've searched the net rather hard but no luck. Here's what I need. I want to build a heavy duty sailboat trailer that's open in the middle. You these in marinas as yard trailers where they can back up to a boat on stands, put cross beams under the boat on each side of the trailer and lift the boat off the stands and drive away. These things cost huge $$$, so I want to modify a heavy duty gooseneck trailer I already own. The problem is finding the axles. Standard trailer axles go all the way across. I want axles that just attach to each side of the trailer arms, thus leaving the center open. The axles are sometimes called walking beam. Other names - types are stub axles or even bogie axles. Weight capacity should be around standard 1 to 3 ton for each axle. I'll add more of them if they're lower capacity. Hydraulic actuation NOT needed.