The newspaper section of the Lincoln Library in Springfield, IL has what you are looking for. Ask for the card catalog of Civil War newspaper letters, which is broken down by regiment. Behind each regimental number are cards giving the name of the newspaper, date of publication, and the name of soldier who wrote. Tha card also gives the page and column. All that is left is to request the microfilm and print the letter. In some instances the number of letters can be overwhelming.
In the past I have written down all the sources and requested the microfilms by interlibrary loan, making the copies locally.