console: help "item name" (the " " tags are good when you need spaces in item names) then use player.additem the code 1 to get it in your inventory. Or
Use COC DBM_Utility2 there is a chest there where you can find all items including Miraaks soul gem. Just be careful as picking up extra items can break other quests. To leave use coc again DBM_test is the entrance of the museum for example.
edit: note to self double check item codes after new quest structure in V17