I bought my LED replacement bulbs at Lowe's a couple of years ago. Lowe's had both MAG-Lite own LED bulb and an off make, I would go with the MAG-LITE own, mostly because it appears to be a brighter LED bulb (Mag lite bulb was listed at 3 watts, the off make at 1, and remember Watt is the measurement of actual power being used, voltage is just resistance and as such must be kept constant within an electrical system, wattage can increase based on how much more power can come through the line).
This has an off make for $19.99 (This is only a ONE Watt Bulb, but as you go down the list you will see FIVE watt bulbs for both AA, C and D Flashlights, but those are $32.00 each).
http://www.theledlight.com/led_flashlightbulbs.htmlThis company is selling a MAG-Lite own THREE Watt Led replacement bulb:
http://flashlightsunlimited.com/magbulbs.htmPlease be careful, remember the two battery bulb is NOT the same as the three battery bulb, which itself is different form the Four Battery bulb. The reason for this is Voltage, the Mag-lite, like all flashlights, are in series, thus Two batteries produce 3 Volts, Three Batteries, 4.5 Volts, and Four Batteries, 6 volts.
The bulbs are COMPLETELY unchangeable between C and D Flashlights with the same number of Batteries per Flashlight, but not between 2, 3 and 4 battery units.
One last comment, LED lights are more forgiving in Voltage then conventional lights, if the light is set for 3.0 volts, and provided 4.5, the light will direct the excess voltage away. This came out of bicycle lights (Especially generator powered bicycle lights). Bicycle Generators have always had a problem with providing excess power and voltage if a bike rolled down a hill to fast and to long. To get around this problem makers of Generator lights installed safety devices in the lights to divert the excess power someplace else. This technology has been around for 75-100 years, it is NOT new. On the other hand LED lights are new, but as part of the build up in light power provided by LED since the 1960s, the technology was applied to them very early, and thus most LED can take excess power the Voltage if so marked. I go into this for the off make Five watt bulb is for 2 and 3 MAG_lite but NOT four. The reason for this appears to be that 6.0 volts of a Four cell MAG Light may be to high, but the 4.5 Volts of the 3 Cells bulb can handle the excess voltage (More probably the bulb is designed for the 4.5 volts of the 3 cell flashlights, so the 3.0 volts of the two cell is within usable limits and most people can not see the reduction in lighting do to the fact the 2 cell can NOT provide all the voltage the bulb can handle).
Last comment, while Mag-Lite is producing a D cell LED MAG Lite, it appears NOT to be selling a C cell LED Mag Lite. Thus your better option may be to buy a replacement bulb.
As to the 3 watt in my flashlight is it more then bright enough, 5 watts will provide only a marginally brighter light. This is a reflection of the differences between measurements of length, area (Square feet) and space (Cubic Feet). One foot in length, brings with it one square foot of area, and one cubic foot, if you go just another foot, the length is just doubled to two feet, but the area is four feet, as is the cubic measurement. Go another foot, length is only three feet, area is nine square feet, but the cubic measurement is even greater, 27 cubic feet. My point is 3 watts provides more then enough lighting, if you go with the five watt, you are almost doubling the light being emitted (Which can be a cubic measurement) but actual greater DEPTH (An line measurement) will be cubic root of whatever is the greater space being lighted by the 5 watt bulb.
Further reading
1 watt LED = 50 Lumens of light
3 watt LED =
5 Watt LED = 140 Lumens
Combination, three LED Three Watt bulbs as one unit = 600 Lumens