There's an excellent source for axles.

Check with any of the mobile home delivery companies in your area.

Mobile Homes are typically built on brand new axles, and those same axles are
routinely removed from under the mobile home when it is delivered and set up on
the trailer pad.

Sometimes the drivers just get to keep the axles as a "tip" and they are
interested in turning them into cash. Othertimes, the delivery company resells
them.

One of the trailer mfgrs up this way will quote a caller two prices for
building a boat trailer, one price with "brand new" axles and another, slightly
lower price with "mobile home take off" axles. Personally don't see a problem
with using the MH axles, but if you were going to build trailers for resale you
would want to do like our local trailer company does and fully disclose that
the axle(s) were previously used to transport a mobile home.