Mythics are available to every player, with a small chance of getting one in every pack opened. It's a very small chance, and it increases slightly in epic and legendary packs.
The key to getting mythics is to just play the game and earn as many packs as you can. You'll get one eventually! The only guaranteed way to get a mythic has been once a year during the game's birthday celebration. For the last two years, Proletariat have given everyone a 'rainbow' pack with one of each rarity inside on the anniversary of the game launch.
Good luck! And remember, epic and legendary units work great. You can get through all the campaign missions (including insane!) without a mythic. Happy hunting!