Suggesting Reviewers: How to find institutional affiliations Many journals recommend that authors suggest potential reviewers. They also often require the institutional affiliation to be listed. However, many researchers who have published in relevant, high-level journals do not necessarily have this information public and some are not affiliated with an institution at all. 

How should I proceed in these situations?
How do you know of these potential reviewers in the first place?

If you know them personally, then you probably also know which institution they're in. If you don't know them personally but have read their papers which are intimately related to yours, then you can look at the first page of their papers to see their affiliation. If you read of them from books, you can still look at the title pages which should list their affiliation.

If they don't have affiliations, then simply write "unaffiliated".