The Combat Helmet is a piece of head wear that goes with the Combat Armor in Fallout 3. The Combat Helmet has a Damage Resistance of 5 with no effects, it can also be worn with goggles or glasses. It's decent for lower level characters in the earlier stages of the game.
It can sometimes be found in lockers across the Capital Wasteland but they're more commonly found on Caravan Merchants. Crow, Moira Brown, Seagrave Holmes and Tulip are all likely to have Combats Helmets to sell. They also sell them pretty cheap, they're usually in poor condition but if you have a high Repair skill you can sometimes buy 3 or 4 for under 100 Caps and then repair them together.