Suspenders were first authorized for the Army in GO 58, 31 July 1883, "according to a pattern in the Office of the Quartermaster General". What that pattern authorized, you can find out by contacting the National Archives or the Quartermaster Museum (link below).
"The Horse Soldier, 1776-1943", Vol. III, by Randy Steffen (University of Oklahoma, Norman, 1978) has a picture of 1901 uniforms (p.77). This was the first time suspenders were authorized to be worn exposed and the caption explains that there was an "inference only' for wearing them exposed.
Any period civilian suspenders may be found in period fashion magazines and catalogues, many of which are reprinted by Dover Publications, www.doverpublications.com.
Good luck in your search,
James B. Ronan II
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