There are radiator shops just like tire shops and they'll be able to point you in the right direction, or even repair yours. Online there are numerous custom manufacturers, like
This oneYou can also google RV salvage and try to find one in a salvage yard.
A few years ago, in the summer, I took a trip up to Fairbanks, AK and all along the way, I had to keep topping off my expansion tank. So I went to a radiator shop there and found that they could remove my rad, rod it out, fix any leak, and replace it for under $300. But they were short of work at the time. Also, they couldn't spot a leak on the rad and told me they didn't think it was leaking.
I didn't get around to it, but years, and a couple-three gallons of wasted 50/50 mix antifreeze later, found that one of the heater hoses from the engine up to the front of the RV had a slow drip-drip-drip leak. Right near the front of the RV. Luckily, diesels all have shut off valves for that at the engine so I drove around for another couple years with no heat. But I'm a snowbird so not a big deal.
Good luck...