Starting with the Confederate Order of Battle, go to U. S. Army Heritage & Education Center website:

http://cdm16635.contentdm.oclc.org/cdm/search/collection/p16635coll9/searchterm/Civil%20War%20Confederate%20Units/order/title

Here you enter the unit you’re searching for and you will find material they hold pertinent to that organization. Here is an example of the documents they hold in their library and are known:

U. S. Army Heritage & Education Center
Carlisle Barracks, PA

3rd Missouri Infantry Regiment
http://cdm16635.contentdm.oclc.org/cdm/singleitem/collection/p16635coll9/id/9229/rec/2