Check out this link for a listing of
monsters by their mlvls, sorted into columns by what act they appear in (from d2data.net...note that I've had to link to the page with frames removed to take you to the right page for lvl 26 monsers, for easier navigation of the whole site, you might want to go to
d2data.net yourself, then click on the monster index for v.1.10 in the left column, then tell it to sort by level).
The earliest mlvl 26 monsters seem to be Stormtree (the thorny giant that gaurds the entrance to lower Kurast), Battlemaid Sarina (who gaurds Lam Esen's Tome in "The Ruined Temple" in Kurast Bazarr), and Mephisto. Notably, however, Witch Doctor Endugu (who gaurds Khalim's Brain in the Flayer Dungeon) is mlvl 27, and the super-unique council members (both the three in Travincal and the three you face @ Mephisto) are mlvl 28. Act 4 is when normal monsters begin to be able to drop the ammy, seems about half of them can, and almost everything in A5 can, barring a few of the monsters borrowed from previous acts. Once you reach NM, everything can.
So I guess the first monster you'll face who can drop it would be Endugu, assuming you play through act 3, instead of getting a rush. If you get a rush, it'll be the three super-unique council members in Travincal that can drop it first. That is, without counting bosses, who get +3 mlvls, and minions, who get +1...probably a few of those can drop it before then, but I'll let someone else figure that out.