If you want a shop really close to BTS Saphan Kai, you can go to Phetboy or K Man Spa.
Boys are often freelance working for several massage shops in the same area.
I've only had 1 massage at that venue, so many masseurs to try.
You can contact the Spa before and ask a boy to come in at a certain time, so the choice is bigger then the masseurs in the lobby.
Thank you vinapu for answering this question on my behalf
My plane on Don Muang airport to leave Bangkok and fly to Bali is scheduled on 12 April in the morning...
Some Line accounts from massage shops:
@onespa
@ btmassage
@home-31
@943mbqak (Phoenix Massage)
@kmanspa
@232uzlha (M-star massage)
You will probably get the Line accounts from massage boys at the end of each massage, if you ask them.