Your supervisor is well placed to give you a different perspective on your work. You know how you’ve thought about and written about your work; they could give you their impressions as a more experienced academic, or share how an examiner would approach your thesis.
Friends and colleagues can offer their perspective on the viva process. Any PhD graduate could share their experience to help broaden your understanding of what to expect. A graduate from your department might be able to give you a nuanced expectation of what a viva is like in your discipline.
Your graduate school or doctoral college can help give you the big picture perspective on the viva. They can share regulations, requirements and as much paperwork as you can manage to show you what the process of the viva is meant to be like.
What other perspectives do you need as you get ready for your viva?