I'm going to show you how to get an ink sac in MCPE. As far as I know it only works in creative mode.
Step 1: Bonemeal & Bow
Shoot the bow and quickly after that tap the bonemeal.
Step 2: Water & Lava Buckets
It also works with water and lava.
Step 3: Results of Water & Lava
And you get these buckets.