Where in Seoul can I find the most concentration of neon lights in the street like I see in Korean movies?
1

bigusredus
The streets and alleyways behind the tourist info place is neon central (can't remember the street for tourist info, but it is right beside the river)
2

amexpat
It's probably harder to find an area that doesn't have a lot of neon signs than to find one that does.
One area with plenty of signage are the streets and alleys around Gangnam station (subway line 2).
Pro tip