On the Recruit screen (look for the Prismo icon) you will find character Pickle Packs for sale.
The "Top Picks" for characters included in the different Packs appear in the upper left portion of the screen. To see all of the characters you have a chance of receiving in a Pack, tap the cards with "?" icons on them. You can scroll through different Packs by taping the arrow icons.
You can buy characters directly (choosing the right bundle for you) with Hero Coins. You can also get characters by earning Pickles as you play - so keep completing those Quests!