How can I find an expert to help review my PhD thesis?
After your supervisor, your best bet is to ask people in your research lab/team if they can review your thesis. A PhD thesis usually talks about specific and sometimes complex scientific concepts. Therefore, even PhD students from other research groups won't have the necessary background to review it. Alternatively, you can ask someone you trust from outside your research lab/team who's research is closely matched to your own.

Note: I didn't include "asking other faculty" because that would be an irrational request (it is not their job and you already have an advisor) unless your current advisor formally suggests this approach.