Maybe try Gemplers or even Kubota itself. They are a standard 8 hole, 8" bolt circle with 6" hole in the middle so anything with that size will work. I've seen all sorts of spacers for cars at Summit Racing so the company that makes those must also make other sizes. Trucks are the same source so that's a possibility as well. But should you? Spacers shift loads and put strain on axles that most manufacturers say not to do.
It's pricey but I had Unverferth make wheel centers for me that worked great.