Jesse Karlsberg has put together two helpful very blog posts about identifying editions of The Sacred Harp. This can be a very involved process, but these will place you at a good starting point.
* The Page 37 Test: Identifying Basic Editions of The Sacred Harp -- "...some guidance on how to identify and distinguish among editions of the tunebook."
* Identifying Sacred Harp Editions -- A helpful table of Sacred Harp editions
The list of Tunebooks, Music Books, and Hymnals by Steven Sabol and Warren Steel will show where some of The Sacred Harp editions are available.