How to Find Peer-Reviewers Who Aren't Busy (During the Summer) TLDR: How do I figure out who to ask to review an article (who isn't too busy)? 

I was asked to submit an article for an upcoming thematic issue for a well-regarded, peer-reviewed journal in my field. Several months after the initial submission, the issue's editor mentioned that they've had issues with finding a peer-reviewer for my article. They asked a handful of people: some have disappeared, some are too busy. As such, they would like for me to suggest names. I don't have an issue identifying people in my field who can review my article. However, I don't know who to identify as someone who would have enough time, especially because it's summer. I know the top people in my field -- the superstars -- but not so much about others I could ask.
You're not being asked to find reviewers who are willing to review your paper. You're only being asked to suggest reviewers. There is no guarantee that any of the reviewers you suggest will actually be the one(s) who review your paper.

Therefore you should not so much identify people who have time as you should identify people who can review your paper. If you're still concerned you could identify people other than the superstars, but suggesting superstars is fine, because the superstars probably know others who can review your paper (so they can suggest someone).