I just now grew an interest in it because of its last evolution's (Shedinja) typing. I haver never heard of a bug-ghost type before and I am interested in finding him. And also, if he needs any items for him to evolve please include it in you're answer.
Nincada can be found at the National Park during Bug-Catching Contests on Thusday and Saturday, post-National Dex. There is a 5% chance to get it at level 26-26.
As for evolution, it evolves at level 20, but you will need a regular Poke Ball in your bag and an empty space in your party to get Shedinja.
Bug-Catching Contest in the National Park (Post- National PokeDex)
Evolution: Keep a regular PokeBall in your bag, wait till your Nincada gets to level 19. When you make an empty slot in your party, then you have my permission to level him up and get your Shedinja.
Source: Bulbapedia/ Bug-Catching Contest
Hope I helped!:)