A good quality cotton, flannel or wool men's shirt can easily be modified to the period look of a four button pullover.  You will want to find a printed shirt with a loose fit. (Buy it an extra size larger)  The print should be pin strips in a mattress ticking style, a small plaid or small check in a muted, drab color.  Avoid loud or bright colors.  If you  find one with out a collar, even better.

Get out your seam ripper and open up the seam that attaches the collar to the neckband. Remove the collar and whip stitch the neckband closed.  Remove any pockets.  Next rip the seam that attaches the button hole tab to the shirt, starting at the bottom and stopping short of the fourth button from the neck. Measure down about an inch and a half from the fourth button hole and cut off the bottom part of the fabric tab.  Fold the two corners to the back, forming a point and sew together.  Sew together the bottom half of the open front of the shirt. The small strip or check pattern will make this seam invisible.  Sew the point down over the top of this seam.  If the shirt has wide cuffs, cut them down.  Turn the edges in and whip stitch the two sides together.

Courtesy of Fannie and Vera's