Trying to form the interrogative form seems awkward and unnatural to me at times.
I think many people try to avoid forming a question here, and would just go with "should"
If we do form a question with it, position the subject after had (hadn't)
Hadn't you better get ready? You'll be late.
Alternatively, I could form an indirect question. Which also sounds natural in spoken English;
Do you think I'd better buy Amy a present?
Don't you think you'd better get ready? You'll be late.
Just some suggestions....