It seems like you just have unfortunate luck. It should eventually come up in one of the merchants inventory.
If you don't want to play the waiting game, use this
save editor and go the item section, use the search bar or manually look up "Crimson Cube", check the box and press "File" -> "Save & Exit", and you should have the cube in your inventory the next time you load up your save ingame.
Don't forget to take your save from the save folder and place it in the root game folder, then do your editing, then place the save back in the save folder, overwriting if necessary; otherwise the save editior is restricted to not being able to edit the Flags and Variables section.