“Outlaw sources” include any objects with at least one of the Assassin, Mercenary, Pirate, Rogue, or Warlock creature types. This means you could spend the mana to activate the ability of an outlaw creature you control or an outlaw card in your hand or graveyard, for example.

You may spend the two mana added by the ability granted by Discreet Retreat on the same spell or activated ability or two different spells and/or activated abilities.

A card, spell, or permanent is an outlaw if it has the Assassin, Mercenary, Pirate, Rogue, or Warlock creature type. It doesn’t matter if it has more than one of those creature types; as long as it has at least one, it’s an outlaw.

Outlaw is not a creature type. If an effect asks you to choose a creature type, you can’t choose outlaw.

If an ability refers to an outlaw or whether a player controls an outlaw, it’s referring only to permanents with one or more of the creature types specified above. Notably, it’s not referring to any spell or card not on the battlefield. However, other abilities may refer to an “outlaw spell” or “outlaw card” in a zone other than the battlefield. Those abilities refer to spells and cards with one or more of the specified creature types.