James - for years the gold standard on buttons was the Alberts book "Record of american uniform and historical buttons" Now the new book by Tice has replaced it for most all collectors. However, both are pretty hard core and delve into many arcane and never-seen-outside-of-the-book buttons.
For just general interest, the "Arms and Equipment of the Confederacy" has a nice depiction of Confederate State and national buttons.
In general terms, Confederate State buttons are so rare for a number of reasons. First, is the simple fact that many of the Southern state buttons were made in the North before the war. When the war bean the supply of any state seals depended on local manufacture or imported versions. (Both had their limitations) Arkansas State seal buttons were only ordered once or twice before the war for example from Horstmann - and there were none imported. Hence they are a crown jewel to dig one.
My best guess is that despite regulations to the contrary - the uniforms of many troops had buttons that were not matched to their state or even their branch of service. A button of any kind, was better than having none on your coat. This is of course different when it comes to the uniforms exported by P. Tait Etc. into the Confederacy that came with branch of service buttons.
One feaure that many miss is the number of Confederates that went with wood buttons or "coin" buttons on uniforms - again, I believe because that was all that could be had.
Hope that this is of interest