do you know any barbershop where locals go,as in bangkok?

Um, the hair-cutting or HE type?

By coincidence, I can help with both answers.

Suthep Road, east of the Nimmanhaemin T-intersection, near Wat Suan Dok is a row of Mens Hairdressing shops.

The other 'barber-shop' is also on Suthep Road, between talad and intersection with Wing 41 road. The wife pointed this out one day as we took a shortcut through their carpark.

Khampangdin road, just down from the Imperial Mae Ping hotel. The name is Pornwilai Barbershop. I used to get my hair cut there every time I visited Chiang Mai. 100 baht for an excellent haircut.

Khampangdin road, near Loi Kroh road, Chiang Mai Barbershop is aircon and does a really good job for 60 baht.

any particular places for a good haircut? im looking at the good old undercut slick back haircut. the last time i went i ended up with slopes on sides and back.