You can find different types of assemblers in scenes. Every scene has its type of assembler.
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.