Start searching eBay and it will be expensive. You need to figure out brand and model.
Bargman L400s (not your lock) have been going for $750 or more for NOS locks for example.
I did a quick scan on google for your style lock. I can't find it. I would contact vintage trailer supply inc, they are on the web. They know locks, may be able to at least identify the model.
In my case, I saved a search on eBay. Eventually, someone listed a NOS
L-400 at a more acceptable price and I immediately bought it. If you find one on eBay, don't wait! Chances are someone else is looking for one too. For some reason, some buyers out there ponder for a while, allowing others to scoop in. To dally is to be sorry.
For now, I would be cleaning, lubricating and probably treating the rust on your current lock. I use dilute oxalic acid to treat rust on vintage bicycles (I'm a collector/restorer).
The "good news" with a bigger diameter lock is you might be able to fabricate an adapter plate (large diameter) to allow a smaller diameter lock installation. Be careful with the length of the latch assembly, you could need a longer one.