You can do the exercises on the Bosu ball. If you can complete the sets with the Bosu down then I would progress to the ball up. It is definitely hard, so I recommend cutting the duration of the holds/sets in half or 1/3. Keep the total time spend doing the balance exercises the same, thus adding more sets (3 sets of 30 seconds becomes – 9 sets of 10 seconds). I do not know where your balance currently is but you should be able to ‘be still’ for at least 10s before adding time (and reducing sets).
A good substitution for a balance disc is a large folded towel or a rolled up yoga/gym mat.