Welcome to FTF.
There are many specialty lights for this, but any small LED flashlight with
the bulb sticking out of the front will do. Simply BE SURE the gun is
completely unloaded and the bolt removed, and shine the light down
the bore. Helps to verify a barrel is clean, free of obstructions, and still
has rifling lands intact...
Holiday season is a great time to find bore sights, just look for any small single led
flashlight, AAA or AA, which is shaped like a bullet.