There will be a sticker and or a stamped number on the back of the axle that you can use to get a replacement axle. You can contact Dexter directly or any trailer supply company who should be able to get you a replacement axle. If you can get the rig lifted up and on jack stands, replacing the axle shouldn't be too hard with basic tools and a torque wrench.
https://www.dexteraxle.com/Contact/Online-Store-Help
or do a search online for Dexter axles for sale. Ordering directly from Dexter isn't usually the cheapest way to go, but they will get you the proper parts.