This is the one of Bryan's many areas of expertise, but not mine. To get you started, I suggest the following web sites for more information on Sebastion County. Good luck with your search.

http://www.couchgenweb.com/arkansas/sebastian/
Sebastian County, Arkansas
Genealogical Homepage

http://www.couchgenweb.com/civilwar/35thf&s.html

"The 35th Arkansas was initially organized at Fort Smith........."
Companies A-D were from Sebastion County, and this web site has a roll of the companies

http://www.hearthstonelegacy.com/sebastion.htm

"Sebastian County Arkansas History and Genealogy including Fort Smith..."