Maglite LED Conversion and Upgrade kits: The best and cheapest dropins and replacements..
Lumencraft LED Conversion Kit Compatible/Replacement for Maglite Flashlight - 800 Lumen - fits 3 4 5 6 D Cell Maglight - High Power SST40-3 Mode Low 45, Mid 315, High 800 Lumens. (800 Lumen Bulb) - - Amazon.com
HQRP Ultra Bright 300Lm High Power 3W LED Conversion Upgrade Bulb for Maglite 3D 4D 5D 6D / 3C 4C 5C 6C / 3-4-5-6 D/C Cell Torch Flashlights - Walmart.com
Dimmable LED upgrade bulb for Maglite 3D, 4D, 5D, 6D Cell 3.6~9V / 10W/ Maglite base - Deshun Technology_Deshun Technology
Lightstar MiniStar30M-EX LED Multi-Mode Upgrade for Maglite (4-6 D Cells ONLY) — CampSaver
Lumencraft LED Conversion Kit Compatible/Replacement for Maglite Flashlight - 3500 Lumen for 6 D Cell - High Power Upgrade with Cree LEDs - - Amazon.com