the proffesor's assistant gives it to you on route 207
As you enter route 207 from the north, the proffesor's assisant will approach you to pick a hand. It doesnt really matter which one you choose, because you'll get the contents of both either way. One holds a Vs. Seeker that will help you find trainers who are ready for a rematch and the other holds a Dowsing Machine.
In the app store
Find poke app 8(berry searcher) by a girl on route 208.
get the Poke tech App Poke happyness
You get the Poke-radar app after you obtain the National Dex.
There isn't one.
the poke radar isn't an app it is a key item, i think you get from prof rowan after completing the national dex
talk to a girl in the eterna city Pokemon center on 1F
Its the Poke'etch app and an app is part of a device called the poke'etch. Apps are things like the daycare checker or trainer counter.
u click and hold the Pokemon with ur stylus
the drowsing machine is a tool that lets you find items that you cant see without it
You can get it from Professor Oak, in Pal Park.
at the poke app store in jubilife city when you talk to the president on the right door.