They're typically expensive, there's not a large quantity out on the market, and because they are reliable and expensive, they tend to be in use for a long time, so many just haven't ever been resold.
Maybe try searching a specific model number or SRS instead of the full name, I've seen some discussions of their FFT analyzers come up a few times and I know Shahriar has used a lock in amp and I think a bench preamp of theirs in videos, and a counter, if I remember right.