Maybe I've misunderstood, but D1-5 chucks don't seem to be an issue. It's other spindle mount attachments that are rare such as collet chucks.
From what I've read, it's not that they aren't available, just limited in choice and pricier than other makes/models not available in D1-5.
Hope someone chimes in if I'm wrong. I've been expanding my search looking at Jet, Kent, Clausing, Birmingham, Acra, and others. Seems everything in the size and price range I'm looking for have the D1-5 spindle and are Chinese manufactured.
Seems a 13" to 15" swing between 36" and 40" lathe not manufactured in China with D1-4 or D1-6 spindle are about as available as hens teeth. There are a few 14x40 D1-4 spindle lathes I've found, but way out of my price range for new. Used ones are rare and always seem to be 1,500+ miles away.