I'm not sure how much help this is but I found Stainless Steel Prep Board is Gauge 20 on Amazon and I saw a YouTube video where they had a 20 gauge stainless steel cutting board.
I would love to see your island when it's completed. Please post the project with how you affix the metal.
I believe it comes premade in sheets for each use.
Auction houses and restaurant supply houses often have new and used restaurant equipment for sale that have stainless components- workbenches, tables, ect. Suspect it will be cheaper than buying new. If it is too thin, reenforce with plywood on the underside.