somewhere on here Peter Jenks put a post about this. If I remeber correctly they are very special & he paid a lot of money to replace them.
As a sporty upgrade I fitted Aldon Automotive rose jointed arms in place of the rubber bushed and worn out arms on my Frog Eye sprite, many decades back.... The rose joints wore out in jig time, compared to the cheaper arms with rubber bushings...
At the risk of teaching egg sucking, IF replacement original design rose joints might be difficult to find, I suspect/hope that if you can not find a direct replacement, there will be an engineering shop that can modify an old style steering joint/track rod end to fit.
It seems over the years Morgan have ever made use of parts found in other cars and folk like Melvyn Rutter, and Lorne at GoMoG.com have spent inordinate amounts of time in research to find original suppliers..
Sorry to ramble on, but hope this helps if you might still be looking for the part(s) you require to keep your wonderful Morgan running..