I know that they used the HAG "Signet" chair for the chairs on the runabout in the six season TNG episode "Timescape" which were then used in Voyager in the conference room. Those were the rounded top version. They also used a square top version of the Signet in Janeway's ready room, and in the TNG movies.
And yes, they are almost impossible to find.
Hag still makes the Signet line of chairs, but they look nothing like the one's used in the Trek series/movies.
I was lucky enough to find two of them over the last few years. Both at thrift stores, and both for under $20.00 each. One was the round top like the one's in Voyager's conference room, and just 2 weeks ago, I found one like the ones used in the TNG movies. It is the top of the line model, which means it has the wider top. But otherwise, it is the same model. And the best part is that it is in outstanding condition for a chair that has not been made in over a decade. I still want to find one like the chair used in the TNG briefing room. So the search continues....
Terry