What is a domain expert ?
When you develop software using Domain Driven Design, the primary focus should be on the domain model, that is a representation of the domain in which the software will be used.
Let's take the example of an accounting software. The domain is accounting, and to build your domain model, you need to identify domain objects (e.g. accounts, debit, credit, postings, ledger, ...) and the rules that govern them (e.g. "in a posting, the sum of all the debits must always be equal to the sum of all credits", or "once a posting made in a ledger, you're never allowed to delete it", or "for every posting, there must be an audit trail of who posted it when").
To develop your model (and your software around it), you need to get the domain knowledge. This knowledge could be extracted from available sources (books, business procedure manuals, ...), but to get it right, you need a domain expert (sometimes called a subject matter expert).
What are the domain experts ?
In the accounting example, the domain expert would probably be an accountant. He/she may probably be or become a future user of the system (some call him/her a "key user" for this reason). But in other cases, it could be a business expert that will never use the system (e.g: a production engineer knows perfectly the production line and the production process, whereas the future users will be the production operators).
Keep in mind that not every user will be a domain expert. A domain expert must have some authority on his domain, so that you can verify with him/her the validity of your designed domain model.
Last but not the least, for many systems, you might need several domain experts to cover the full spectrum of the domain, because nobody can know it all (e.g. for an ERP you need domain experts from all over the company: sales, procurement, accounting, production, warehouse, ...).
Where to find the business experts ?
It's difficult to give a universal answer. I can only tell you what worked for me: I think the best way to find them is to ask the customer of your future system.
Ask for an expert who can describe what the system shall do, who can explain how the business works in the considered field, and who could answer the more complex questions you could have about the topic.
Ask for someone who has authority to approve orientations and choices about how the software fit the domain needs.