FamilySearch Indexing: US, District of Columbia—Freedmen's Bureau Letters, 1865–1870, Additional Helps
Google has published a pdf versions of the Official Army Registers for 1866 and 1868. They are available online for free and are searchable. An alphabetical listing of officers are at the end of each book. I find it very helpful to verify the correct spelling of officer names when they are hard to decipher.