Or consult the Company of Military Historians:

http://www.military-historians.org/

For example, I know that the CMH has a plate showing the post-war 69th New York Infantry's uniform from about the 1880's, and if I remember correctly, the old 79th New York did maintain some vestiges of their pre-war Highland uniform, but there is little documentation that I am aware of concerning the use of Highland "kit" in the American military, save the 79th.

Post Civil War many of these units broke apart or reformed into Westward heading Army units.
Individuals who had served in state volunteer units did join the regular army for post-war service in the West, but since State militias could not be federalized (not until Congress passed the Dick Act in 1902), the only militia units on duty in the West were from Western states & territories.

Remember that the various state militias in the late 19th century adopted their own uniform regulations, and that many individual units -- the "chum and chowder societies" did as well, so you see all sorts of variations of kit, including the blancoed pith helmets (very popular in the regular army in the West as well), etc.

What sources have you found for some of the uniform "kit" you describe?

Now, there was a fraternal organization, the Order of Scottish Clans, that did wear Highland garb on a frequent basis, and of course, the several St. Andrew's/Caledonian Societies and pipe bands, but I can't say I have seen very much concrete evidence of a large number of kilted soldiers in the American militia. If you have a "smoking gun", I'm sure a lot of my fellow rangers and historians at Civil War historical sites would love to see it!

Regards,

Todd

Seasonal Park Ranger, Wilson's Creek National Battlefield, NPS