to get the colours of uniforms, and flags.
I have a nice illustrated series, called battlefields of the civil war, commanders of the civil war and fighting men of the civil war, from the yankees and rebels series, and they are abundant with illustrations and pictures of uniforms and equipment. Otherwise, I'm sure the osprey range of books will have something suitable - assuming you just want the looks. The three books I have are full of information on the actual war as well.
You can find plenty of flags on the web.
Many states preserved their flags and now pictures of them are available online.
e.g. Pennsylvania:
http://cpc.state.pa.us/cpcweb/flag
many thanks guys :)