I need to obtain a 21" Radius Grayscale monitor to replace one that has died. It is the display for an Apple IIsi.
I have some old Radius B/W monitors, although I'm not sure if any of them are precisely what you are looking for. If you are still looking for one, contact me, and I'll look more closely and let you know exactly what I have. If you (or a friend) are anywhere near Boston, you can come pick up any of my old stuff for free. Packing and shipping monitors is not my idea of a fun thing to do.
On edit... These are three Radius TPD/19 monitors from Mac IIsi machines. Any and all all free for the taking.
On further edit... A collector of antique Mac gear has taken the IIsi machines with the Radius TPD/19 monitors, but will be glad to pass either of them along to you if you still want or need this unit. He collects things mostly for display, not for actual use, so he only took the Radius monitors reluctantly, since they matched the Radius cards in the pair of IIsi machines that he took.
I also have some other (non-Radius) monitors that might conceivably work, including some very large DEC fixed-frequency monitors with single BNC connectors (and some with three BNC connectors).
I also have some Radius Pivot monitors with standard Mac-style video connector cables.
Everything free for the taking in Bedford MA.