I've been hunting for unique items with the most magic slots and I'll be showing the ones I think offer the best options with the most magic slots.

Found a Yuzan with 31 available magic slots. The 1 slot is occupied by a blue soul shard. Others I've found had 1 red or gold soul stone. I've never found one with 32 slots or with a gaki bone.

Found a Kobara with 23 slots (24 max). 1 slot comes with red soul stone. This armor is ideal for the wizard, although I prefer to get one with a blue soul stone, which I'm yet to find.

Hwa Rang Chil-Ji-Do with 31 slots. 1 slot comes with green soul stone. Best one I've gotten so far.

Ryushi bow with 30 slots

I've also found Hidari-No which is 1h sword with 30 slots and some polearms.

Got your armor with 31 slots as well.