Yeah, the easiest way to find fragments will be burrows. They can be hard to spot, but they are fairly common. Look for a 3 block wide hole, easiest to spot on a plains or other flat-ish treeless biome. The two outer blocks are on one deep, but the middle one drops straight into the burrow, and you can see the chests from the surface. There are 6 total chests in the burrow, although only 3 are visible at first. You should be able to figure out where the 3 others are and how to unlock them.
There are two more structures, caverns attached mostly to ravines, but they have cloak blocks which are incredibly hard to spot. Much easier are the ocean temples, though they appear to be fairly rare. They are also the most risky, but worth it because you can easily walk out with a ton of fragments.
Speaking of the fragments, you will end up with blank fragments occasionally. This is not a defect, but that means you have reached the end of your knowledge of the world. Stash them in a chest until you do more unlocks.
There are 100+ fragments, and you find them in groups of 5 to 10, ish, in Chromaticraft structures and 1 or 2 in loot chests around the world. This means a lot of exploration is needed. You'll be doing at least some exploration anyway for Essentailcraft and even immersive Engineering, for the Arc Furnace blueprints.