What Impression to Use

[ds1999]

Hi all, I have been wondering for a while on which impression to use for events when given the unit to be portrayed. I have had this problem before and would like some advice on where to go for information. Our unit will usually portray Wisconsin units for example during our events and I would like to know where to go to find what the troops of a specific unit wore at any given time during the war. I know that photographs of members can help to an extent, but in some cases there are none. I am just wondering if anyone could give me some tips on where to find this type of information.
Thank you in advance,