Many online vendors sell Photoresistor light sensors for $2 or $3 each.
I found a cheap source of them in my hometown. Go to a dollar store and find an auto “Nightlight”. Those are the small lights that plug into your wall socket and turn on automatically at night so your small children do not get afraid of the dark.
Only $1 each at the dollar store and contain a photo sensor. Remove it and use it in your own projects.