**Use industrial scientists.**
I don't have exact figures, but industrial scientists outnumber professors by what, 20x? 40x? Maybe even more? Whatever the number, all that manpower is available and yet it is seldom tapped. I am a outlier in that I referee for the Royal Society of Chemistry (RSC) but I have never encountered any other industrial scientists who referee papers at all for any publisher. Why is that? Cynical thoughts would be that academia doesn't want to dirty itself by working with "prostitutes of science", people who weren't "good enough" to make it as professors, or people who wouldn't be interested in research that doesn't have immediate economic applications. Or is the thought that because industrial scientists don't publish much compared to their academic counterparts, they aren't interested in what's in the literature or making sure that it is kept to high standards? Most assuredly, they are very interested. Some industrial scientists will say no, but that also happens with professors that are asked.
If I were working for journal and needed to find referees, I would start trying to tap this potential workforce now. Start with the few people you do know that are in industry and ask them who else they can recommend. And ask those recommendations for additional recommendations. And then do something with them. Start sending them manuscripts, even if you already have your 3 reviewers lined up. Take these new reviewers out for a test spin and see how they stake up to the regular reviewers. Once you have confidence in them, then start using them and give your regular reviewers a bit of a break. And by lining these new reviewers up early and using them, you are more likely to have their loyalty down the road when other publishers approach them.
When the RSC first approached me to referee about 10 years ago, I was honored by the request and have been happily helping them out. I contacted the American Chemical Society (ACS) shortly thereafter to volunteer my help. They said they would be in touch but have never yet sent a manuscript to me. So if the RSC and the ACS were to both approach me for refereeing and I only have time for one, who do you think I'm going to help?