Shadowmere in The Elder Scrolls IV: Oblivion is found at Fort Farragut, which is located just east of Cheydinhal. You receive Shadowmere as a reward from Lucien Lachance after completing the Dark Brotherhood quest called "The Purification." After completing this quest, Shadowmere will be waiting for you outside the main entrance of Fort Farragut

. If you lose Shadowmere, she typically returns to Fort Farragut after some time, unless she is equipped with horse armor, which can cause glitches preventing her from returning. In that case, she might be found at the last place you left her. Fast traveling to Fort Farragut and checking near the entrance is the best way to find her again. On PC, console commands can be used to spawn or teleport to Shadowmere if she cannot be found

. In summary:

• Shadowmere's main location: Outside Fort Farragut near Cheydinhal.
• Obtained after completing "The Purification" quest in the Dark Brotherhood questline.
• If lost, check Fort Farragut or last known location; use console commands on PC if necessary