I am not aware of where to find voter lists for St. Louis during this time period. As an alternative, let me suggest city directories which are available here on ancestry.com.
I have had some luck finding relatives in St Louis using these records, although they are not well indexed (you have to scroll through a lot of pages looking for the person you want).
It looks like ancestry has a partial directory for 1863 and a pretty complete one for 1864. That may help you find what you need.