There are a number of organizations you could contact regarding this.

I would first try the corporate archives of Macy's and JC Penney. Knapp's and Hudson's may no longer exist, but their assets were acquired by other department stores. The bulk of the J.W. Knapp Company's holdings appear to have been acquired ultimately by JC Penney, but you will also want to contact the Charles Stewart Mott Foundation, which held some Knapp's properties at one point. Hudson's became Marshall Field's briefly before that company's purchase by Macy's. These companies should have retained some, if not all, historic corporate records.

Another possible source would be any educational institutions or professional organizations (like unions) with which your grandfather had an association. This could be as a student, instructor, or member. Schools often archive materials related to alumni and teachers, while a professional organization might have compiled publicity materials.

The last option would be newspaper archives and public institutions like the local library in Lansing and Detroit, the Library of Michigan and the Michigan State Historic Preservation Office. The state library has archives, including photographs, relating to the history of Michigan's department stores. The SHPO, while primarily dedicated to more fixed aspects of the built environment, has a program called Michigan Modern that encompasses all aspects of mid-twentieth-century design. As a start, you could try consulting their publications: Amy L. Arnold and Brian D. Conway, Michigan Modern: Design that Shaped America (Layton, UT: Gibbs Smith, 2016) & Brian D. Conway, Michigan Modern: An Architectural Legacy (New York: Visual Profile Books, Inc., 2018).