It's the essential mid-game pistol, paired with the .44 revolver; the Eagle has a longer range and higher damage, but the revolver has a lower AP cost. Not much is going to be along to replace either pistol until you start finding Gauss pistols, especially since AP .44 rounds are rather common, even in the end missions.
Locations[]
Sold by the Brotherhood after finishing Macomb and Preoria.
4 carried by beastlords in Quincy (one carried by the Beastlord commandant in Hillary Eastwood's home. Also carried by the Beastlord in the northern sniper tower on top of Quincy's detention center). You could also Steal one from Elliot Nesting.
The two stairwells in the main Mardin graveyard are both guarded by a single beastlord each. Each holds another Desert Eagle.