I have bought a TON of Civil War era repros at
http://whittlesfabrics.com/cgi-bin/Store/store.cgi - and look for their backgrounds bundle on the bundles page - 10 FQ's of different shirtings and cream/tan prints.
You have to do some hunting, but the same fabrics that sell for $12-15/yd in the quilt shops sell for $4-6/yd at Whittles. :) And they've got Civil War up to the rafters. :)