You can print the sheets from IMSLP for free:
https://imslp.org/wiki/12_Etudes_in_all_the_Minor_Keys,_Op.39_(Alkan,_Charles-Valentin)
If you want a hard copy, go to Sheetmusicplus.com and buy the Etudes in the Minor Keys Op. 39, Book 1 (The Symphony is etudes 4 through 7 but you also get the first three etudes). This is by the French publisher "Gerard Billaudot Editeur" and is very expensive ($64!), but it's a real collector's item.
https://www.sheetmusicplus.com/title/12-Etudes-Dans-Les-Tons-Mineurs-Op-39-Vol-1/1975219There is an American publisher called Masters Music Publications Inc. that prints (as far as I know) the exact same edition of Alkan's music, but you can buy them for much more inexpensive. However, as far as I can tell, they have not published the 12 etudes in the minor keys Op. 39 (or even the Symphony by itself for that matter).
I've been searching to see if I can find where to buy a hard copy of Marc-André Hamelin's reprints of the Symphony, but 10 minutes of Googling didn't render me anything. Maybe I just looked in the wrong places.
I have a hard copy of the French publisher.

Fortunately, I bought it from a music store at Indiana University on clearance along with the Sonatine, Esquisses Book 4, and Cappricio Alla Soltadesca. Combined it all cost me $20.
