Websites like Modern Chess often include standalone PGN downloads with the purchase of their calculation and endgame courses.
A super-intensive calculation workbook designed by Olympic gold medalist Sam Shankland. It covers deep calculation, candidate moves, and defensive resources. Download Calculation Workbook for Tournament Players pgn
Sites like PGN Mentor offer massive, free archives of specific players and classic tournaments. You can download these files and "mine" them for tactics to calculate. Websites like Modern Chess often include standalone PGN
