I have an answer, but you won't like it. I found Garrison's location in the southeast corner of Christian County on Swan Creek, but no such village exists on the highly detailed 1876 Campbell's Atlas of Missouri. This leads me to assert that the village of Garrison did not exist during the Civil War. I could be wrong, but that's the way it appears to me.
If it is any consolation to you, Christian County was the scene of lots of small-scale Civil War actions, and I have seen Swan Creek mentioned several times in my Civil War research over the years.