Stalkers can be found in the alternate dimension known as "Grayspace"

Tower of Fantasy players will face a mission in the museum to collect data from Stalkers located in Grayspace. This is the quest called "Visitors from Outside".

This quest requires you to defeat the different types of Stalkers in order to succeed in the quest tasks.

Released in the Vera 2.0 update, Grayspace is huge. It's an alternate dimension that has its own exploration progressions, as such players often don't know how to.