Hi Dennis, check out this product. Back in the day they were made primarily for developing finger strength and dexterity, but the new models included simulated strings. Here is the description they include: “By removing the reversible molded grip you can uncover a simulated string that helps develop and maintain finger calluses for string musicians.”
I had one years ago, but it didn’t have the simulated string feature, so I wouldn’t be able to comment on how well it works.