I’ve seen a fake one on Acoustand they were designed to look identical on the picture, but was made without metal frame. Anyway later i realized they got nothing is stock and everything made in order, guys are very slow. You may find something usable
on their site.
If you need the original Technics armboard they are $200-300 minimum.
I think you could order a metal arm board instead.
A friend who left me Obsidian plinth asked local vendor to make wooden arm boards, they must be pained just like the obsidian surface. I'm gonna get them soon.
Another recommendation is to replace stock footers from your obsidian plinth and use Audio-Technica
AT-616 pneumatic insulators (they are rare, but the best). This is
how it works.