I'd thank them in the acknowledgement section. I think some (most?) journal will allow such a section, and most people use that to thank their research funding sources and/or collaborators who had not made the list of authors.
Andy
You do need to get permission from people to include their names in an Ack. section, usually. Referencing as a personal communication is perhaps better.
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Frank E Harrell Jr Professor and Chair School of Medicine
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