In Borderlands 2, the Bonus Package is obtained randomly from any suitable loot source, but has an increased chance to drop from Boom and Bewm located in Southern Shelf.
In Mr. Torgue's Campaign of Carnage, it can be purchased from a Torgue vending machine as the item of the day, for a price of 613 Torgue Tokens.
In Borderlands: The Pre-Sequel, the Bonus Package is obtained randomly from any suitable loot source, but has an increased chance to drop from badass torks.

Special Grenade Effects

2x more awesome, bonus extreme! – Increased damage. When child grenades explode, each releases one additional child grenade.

Usage & Description

The Bonus Package is a powerful MIRV grenade that launches child grenades in a standard MIRV distribution, and then the child grenades each eject another child grenade of their own. Its multiple series of child grenades allows it to spread explosive destruction over a wide radius, possibly destroying entire mobs of foes with one grenade. In enclosed spaces it can cause phenomenal amounts of damage where targets can sustain multiple hits.

Notes

• The Bonus Package is compatible with all types of delivery mechanisms.
• The Bonus Package can drop from either Boom or Bewm and, as a result, it is possible to have two drop on a single run, making the Bonus Package one of the easier legendary items to farm.
• The Bonus Package has a Seraph counterpart called Meteor Shower, which has a generally higher number of child grenades doing more overall damage in a tighter radius.
• It is recommended to obtain a Bonus Package with at least some fuse time, as otherwise the excessive friendly fire damage can potentially cripple the character using it.
• A homing, sticky homing or rubberized Bonus Package is also potentially dangerous to the character using it if their target is in melee range, as it will likely explode before they can withdraw from the vicinity.

Media

Borderlands 2 - Sticky Homing Bonus Package (Legendary Grenade Mod)