Leather is needed to spawn cows that would give me leather to spawn cows that would...
start fishing and breed Mariculture Cod to smelt in the crucible smeltery to give 11% chance of leather per fish
build a grass pasture and hope for cows. remember to account for biomes. you can build the pasture in a mushroom biome and shear the mooschrooms that spawn as well.
build a Bloodmagic alchemy set and use 3 rotten flesh, 1 flint and a bucket of water. for 3 leather
Head to the nether and kill imps for their skins. Not recommended due to difficulty of the nether in AS.
1Along the same line as the mushroom biome bit, it might be worth mentioning that animals don't spawn in Ocean, Desert, Beach, or River biomes. Any other biome should be good.– Unionhawk ♦Apr 25, 2014 at 4:29
For me personally the best solution was to go kill imps in the nether, they don't fight back and I believe 4 imp leather make 2 and they tend to drop 2-3 imp leather, so it's pretty easy to do.
Build a drying rack (3 wooden slabs in a line I believe) and hang rotten flesh from zombies there, after a while it will turn into Jerky for food. Do what the people above said.