Paper

Paper is a particularly useful material, being part of the crafting recipe for Banner Patterns, Name Tags, Books, Maps, Cartography Tables, Firework Rockets and subsequent corollaries of those such as Bookshelves, Enchanting Tables and Written Books. 

There are, by and large, two very clear ways of obtaining paper: Sugar Cane and Shipwreck Chests. You can also obtain paper in Village Cartographer Chest and Stronghold Library chests. 

SUGAR CANE

Generates in patches of 2-4 block high columns (average height 2.5 blocks) in most Overworld biomes except most Mountainous biomes, Mangrove Swamps and all cave biomes. However, the generation requirements exclude certain biomes (for example Stony Shores). 

It can generate as long as there is a certain block (Grass, all types of Dirt, all types of Sand, Mud, and both types of Moss) that is horizontally adjacent to a Water source block (or Ice as can be seen from Frozen Rivers), though it is more common in certain biomes than others. 

Deserts have the highest chance to generate Sugar Cane, followed by Swamps. All types of Badlands generate Sugar Cane a bit more commonly and are followed by the rest of the biomes. When Sugar Cane generates next to Ice, it breaks and falls as an item, meaning if your simulation distance is high then it will despawn after 5 minutes (yet another reason to have your sim dist low by default). 

If you are especially desperate, Wandering Traders have a 6% chance to be able to trade you 8 Sugar Cane for 8 emeralds.

PAPER

Shipwreck Chests (Supply and Map), Cartography Houses (which village type?), and not really relevant but Stronghold Library chests.