This is one project that I don't really feel the need to purchase a pattern. There are books with patterns, as well as an excellent one online at http://www.18cnewenglandlife.org/18cnel/shirt.htm and http://www.18cnewenglandlife.org/Hive_project_series%20Shirt.htm . I have a couple of regency shirts in my collection that I will be looking at closer later in the year.
Shirts were worn for day and night wear and were washed a lot. Linen shirts were popular, and many fastened with dorset buttons - one button at the throat and one at each cuff. To make dorset buttons, just take somewhere between one and two yards of heavy thread (buttonhole twist, for instance) and wrap one end around a pencil or knitting needle several times, then thread the other end on a needle, carefully remove the coil from the pencil, and sew buttonhole stitch around the coil until you run out of thread. The last few inches of thread is used to sew several stitches across the back of the button.