Below is a guide on where you can find Strike Pieces in Horizon Forbidden West.
Common
Burrower Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: The Daunt
Grazer Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong
Leaplasher Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong
Scrapper Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong
Scrounger Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: The Daunt
Uncommon
Bristleback Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: The Daunt
Charger Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: The Daunt
Clawstrider Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong
Fanghorn Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: The Daunt
Glinthawk Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong
Longleg Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong
Plowhorn Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong
Skydrifter Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong
Widemaw Strike Piece – Purchase from the Strike Carver at the Hunting Grounds: Plainsong