Its blueprint can be found in a secret area in the Ancient Sewers, hidden under a pool of poisonous water. The blueprint can either be accessed by diving through the pool of water, at the risk of taking damage, or by using the Homunculus Rune to reach it.
The pool appears to be significantly deeper than most other pools and will have a bottom made of false ground that you can pass through. Once the pool is passed through, the blueprint will simply be sitting on the ground of the square room, along with a return stone to escape the room.
The Frontline Shield has a long history in Dead Cells based on the responses of players, rather than game lore.
Back in Early Access and in v1.0 of the game, the Force Shield used to have the name currently used for this weapon, and couldn't parry but blocked more damage than other shields when held up. Because of this inability to parry, the Frontline Shield was one of the most hated items in the community. Thus, in v1.1 the Front Line shield was reworked into the Force Shield, which is able to parry, and generates a force field (making the player invincible) when held up. Then, in v1.2, after months of insistent requests for a Brutality-scaling shield, the Front Line shield was revived as a completely different item from the original.
As a joke reference to the long process that led to this, Dead Cells lead developer deepnight (Sébastien Bénard) specified in the flavor text that the Front Line shield was born from "the tears of the community", referring to the significant negative responses that caused the general rework.