Besides this site there's Vintage Trailer Supply, Facebook: Vintage Trailer Parts Swip~Swap, Ryan's Vintage Trailers and Parts, among others. There's also VintageCampers.com, ebay and of course Chris at firstname.lastname@example.org. He often works miracles finding parts for these old trailers. Good luck!
If you watch eBay and Etsy, you'll see vintage NOS (New Old Stock) cabinet hardware come up for sale from time to time, sometimes with very large quantities available. Mid-century stuff is pretty popular right now, and a fair amount of hardware has come out of the woodwork. There's also new hardware out there in similar styles, if you happen to find something you can live with.
The pulls in trailers in the 50s and 60s were sometimes the same as ones used in the post WW2 building boom. We found original boomerang ones in a store in Portland, OR that carries some new and many more vintage/antique doo dads for homes. If you don’t have any luck elsewhere, you might consider visiting a local architectural salvage place. They can be treasure troves. Some even have their items online with pics, dimensions, descriptions. This is convenient, but not nearly as fun as rummaging around.
This is a vintage NOS pull I found for my '57 Cardinal, pretty typical mid-century style. I was looking specifically for a copper tone to match the original light fixtures, else I could probably have found a new chrome pull that would work.
Here it is on the cabinet on the left, original plastic pull on the cabinet on the right. Several of the original plastic pulls were damaged.
If you're close to a gentrified city or suburb, check out the local hardware stores.
A few years back I was working on fixing up my grandparents house to sell and found that the neighborhood hardware store was a treasure trove of vintage goodies. Many new as still in production new. They obviously catered to the market which is something the big box stores don't.
As a an example, old style mortised door latches were in stock and matched the original pieces in the house. The house was built early 20s.