How did they know that some Psychobuster still existed?
They are not sure that there are any weapons left, but the information regarding the existence of the Psychobuster itself is given to them most likely by a False Minoshiro.
As seen in the next episodes, it also helps them navigate to the presumed location of the Psychobuster.

It all about the hope of finding one. Similarly, the Queerats previously attempted to search for weapons to use against the cantus-users, with Kiromaru willing to sacrifice a large part of his expedition.