You will need at least one shirt to complete your uniform - two or more if you want to attend multiple day events. There are many options to select from. You can either buy a military issue shirt in white muslin or gray wool, or you can go with a civilian shirt in any period fabric you choose. Most of the time your shirt will be completely covered up by your jacket. But when you are lounging around camp on a hot day, you'll likely be in your shirtsleeves - so you'll probably want it to make an impression. The shirt above is made of pillow ticking. Some fabrics are much more vibrant - bright paisleys, checks, plaids or solid colors. Have fun with it!
Just about any period shirt will work for either a Federal or Confederate impression.
C & D Jarnigan & Co. (2000 prices)
#812 Issue Pattern, white muslin $27.95 #817 Civilian Pattern shirt, cotton colors $31.95
In addition to machine-stiched shirts, it is also possible to purchase shirts with handsewn seams. These can run as much as $100 or more, but have a more authentic look to them and can be made to order in your favorite reproduction patterns and colors. One source for extremely authentic shirts is C. J. Daley: www.cjdaley.com.