You have a few options in that vein. The first is to remove the hubs and take measurements of the spindle with a dial caliper to determine what hubs will fit. There is a chance that a standard trailer hub, with a trailer wheel bolt-pattern, will fit.
If that one does not pan out, you can replace the axle assemblies with trailer axles that use a more standard hub and wheel.
Finally, and I hesitate to recommend this, you can have the spindles cut off the axles and weld new spindles on. This operation must be performed by a certified welder. The spindles have to be welded 100% square and concentric to the axle, if not, the trailer will not ride right. Best case this leads premature wear of suspension, tires, hubs and bearings and the worst case is catastrophic failure of the axle assembly.
Patrick B - 4/11/2013