Currently, I don't see any way to get deepslate (it may be in the wandering trader pool) but I think this is a very requested building block and I would like to see a way to obtain it that's always available, I have 2 ideas of how this could be implemented:

-add it to warp grass in a similar price range as terracotta
-add a custom crafting recipe, I believe 8 cobblestone around 1 ink sac for 1 deepslate block sounds fair and balanced

And while we're talking about deepslate and custom recipes, it would be nice to see custom recipes for all the deepslate ores like the ones we currently have for normal ores but with deepslate in the recipe instead of stone.

EDIT: deepslate confirmed to be currently in the pool of the wandering trader