I tried to find one a few months ago without any luck unless I wanted to pay silly prices at specialist places.
In the end we bought a cheap as chips (£12.99) mount from amazon and then used the plate from it on the mount we already had in place.
The old plate and new swing arm mount from amazon went up to the bedroom with the old TV.
The original swing arm mount we had in the main room cost a fortune 12 years ago but was only a 200 x 200 vesa and the new tv was 400 x 400.