There are 2 short time hotels in Nana Plaza & 2 on soi Cowboy (some people say there are other ones but they are more difficult to find and I did not try them yet).
Nana Plaza: the 2 short time hotels are both on the third floor. One is on the left side, next to the elevator. The other one is on the far right, next to Cascade Ladyboy bar. Both cost 300 baht for 1 hour, but the first one (on the left, next to elevator) is much cleaner, beds are in better condition, the staff is more friendly and even the condoms are cheaper! (20 baht versus 25).
Cow Boy: there is one short time hotel right on soi Cowboy. It is called Penny Black and is next to Raw Hide Gogo bar on soi Cowboy. The rooms are expensive (450 baht) but clean and rather large. The other short time hotel is on the right when you exit soi Cowboy on the Asoke side. 10 meters from soi Cowboy, at the top of small stairs. Not easy to find but the girls know. Rooms are 350 baht there and a bit smaller but still quite big compare to what Nana Plaza offers. I didn’t buy condoms there yet…