Edge of Eternity

There will be chests scattered all over the game, easily enough to get 100 before you reach the temporary "point of no return" for Herelsor, Jagholm, and Inel at the end of Chapter 2.

Items dug up by your Nekaroo also count for this, so if you're close to 75, run around on your mount (the only way I know to check is to accept the mission to open 100 chests from Lonesome Stranger by Herelsor Hill Inn, which will show you how many out of 100 you've opened in the Quest tab).