Where do I get advice and guidance from in my PhD if my supervisor is not an expert in the field I am working on? I decided on my supervisor because he is the only professor working in this field in my country, and he is very resourceful. He helps me out a lot on every aspect other than technical guidance because he is an experimentalist and I am focused right now on numerical simulations. How do I find resources other than research papers for guidance on my PhD
From your question, I understand that you should be on good terms with your advisor. Then, I think you should be straightforward and ask him to connect you to some other experts in the field. Perhaps, you could even have one of them on your committee as your co-advisor.
You could probably ask the other professors at your current or even the previous department for some help. Personally, I'm always happy to help students previously in my classes.
If your university regulations permit, you could take a short sabbatical (usually 3-6 months) in another university under the supervision of a professor expert in your desired field. If you are in the early stages of your Ph.D. studies and the regulations require you to be a 3rd- or 4th-year student before taking such a leave, you could still talk to your possible choices and seek their help. Then when you officially take the sabbatical, you could work closely with them.
There are lots of online groups and websites dedicated to helping people having questions similar to yours. Being an active member of them would benefit you greatly in the long run.