Inexpensive bike lights are available at a lot of random stores, but they are more trouble than they are worth. Most cheap bike lights do not produce enough light to properly illuminate the road in front of the cyclist. This doesn’t mean that good quality bike lights can’t be found for low prices. Rather, not all bike lights are created equal, so be careful about what kind you buy.
The most important factor in bike lights is STVZO. This is the standard for brightness and lighting pattern. An STVZO rated light will illuminate the road in front of the cyclist, and not the passing trees or buildings.
It will also not blind other road users. Cheap, unrated bike lights have very wide, unadjustable light patterns. These shine light right into the drivers eyes. Drivers have multiple levels of brightness at their disposal and tend to only use the bright lights when in extremely dark areas. When they see someone coming, they dim the headlights. Cyclists do not have this ability as their hands are busy else wise. So, good bike lights need to be bright enough to aid in nighttime vision and angled accurately so as to not blind oncoming drivers.
So, always look for STVZO rated lights . Otherwise you will be paying to light up the pavement and not actually facilitating your own cycling. STVZO means safer and more confident cycling.