If you can't get some rod from fogducker, Fastenal on Industrial Ave beside the Harley dealer will likely have both rod and bolts in that size. They prefer to sell in bigger batches but will sell a single bolt that size if you ask nice.
How close to being .590" do you have to be? If you've got to be real close, it might be better to drill it undersize and then ream it to correct dimension. Or you can drill it undersize and then drill it to .590 and it should be within a few thousands if your drill bit is decent.