Question

I need some good dark types or ghost types in Pokemon alpha sapphire?

Answer 1

It would help to know what role you were thinking of covering with those Pokémons and if you wanted them monotype or dual type.
I made a quick check of what 'mons are available in OR/AS
DARK
Mightyena
Absol
Sharpedo
Cacturne
GHOST
Banette
Dusknoir
Froslass
SPECIAL MENTION
Sableye
If you want just a Dark type, Mega Absol is your only choice, otherwise, Mega Sharpedo is fun to use (click the name for moves)
take a look at those move sets for Absol:
Tactician
Absol @ Absolite
Ability: Super Luck / Magic Bounce
Hasty / Naive Nature (Preference)
252 Attack / 4 Sp. Attack / 252 Speed
- Protect
- Sucker Punch
- Fire Blast
- Play Rough

Magic Sweeper
Absol @ Absolite
Ability: Super Luck / Magic Bounce
Jolly Nature
4 HP / 252 Attack / 252 Speed
- Swords Dance
- Night Slash / Sucker Punch
- Play Rough / Psycho Cut
- Play Rough / Superpower / Night Slash

For GHOST, the choice would go between Banette or Dusknoir, but they cover different roles, so look here and make your choice.

Lastly, and this is personally my suggestion, if you want the best of both worlds, I'd go with Mega Sableye.
With just one weakness, Sableye can become a real thorn in your enemy's side.
Take a look at his Smogon page, and pick the moveset you like best!

Hope this helps dude!

Answer 2

Dark
Early game, Mightyena is pretty decent.
Later on, Absol is your best bet. Especially if you need a Mega.

Ghost
If you need a wall, catch a Duskull, evolve it to Dusclops, and give it the Eviolite. One of the best walls in all of Pokemon.
If you need a more offensive option, or a Mega, Shuppet is a great choice. Especially Mega Banette. Prankster is awesome.

Source(s): Experience.