Depending on the red dot, like the Aimpoint Micro series, the battery will last for several years with the unit turned ON.
Getting a battery(batteries) should be no problem.
"If you look through your scope and see your shoe, aim higher." -- said to me by my 11 year old daughter before going out for hogs 8/13/2011
My Hunting Videos https://www.youtube.com/user/HornHillRange