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.
The two stairwells in the main Mardin graveyard are both guarded by a single beastlord each. Each holds another Desert Eagle.
On the Springfield mission, near the end is a building across the street from the church with two ghouls inside. The ghoul next to the red refrigerator has a crate behind him, in it is a Desert Eagle and some ammo.