On the map, next to the photo of the scene, you can see what type of assembler you can get in that scene.
In the description window for a scene, you can see the assembler items you can get in that scene.
In the Collections window, tap an assembler listed under the requirements, then tap the Show Me button. A marker on the map will show you the scene where you can get that assembler.
You can also buy assemblers in the Other section of the store.