Volleyfire Lights from Warlord Games
They take a standard quarter sized battery that slides into the housing from the side. You can glue, paint and static grass right over the circuit board. I mounted mine to a mapboard (hard cardboard) base although you could use almost anything as the circuit board will keep it from curling up (which is common with cardboard that gets glued/painted). Each unit consists of the battery housing, on/off button and six LEDs mounted on a circuit board strip. Warlord is even nice enough to include the batteries!
They have three settings. There is a little push button that turns it on. The first setting is volley fire where the lights will all flash at or near the same time with a 4-5 second pause in between to represent massed fire. The second and third settings (just press the button to cycle through the settings) are - as I recall the official description - fire by companies and independent fire. To be honest, to me, I can't tell the difference. Both settings randomly flash the lights to represent sporadic fire. You can see in the picture below I managed to catch it just at the right time to show the "volley fire" setting.
You can buy these from Warlord Games, here is the link to the item in their online store - Volleyfire Lights. They include a YouTube video in the product description which is better than anything I can do myself.