You can book interpreters through the Gangnam Medical Tour Center, or through Seoul City Office, but if you use these places. The clinic has to be registered with them, sometimes small local clinics are not. Some people have also had some luck using My Seoul Secret. They are an agency, but they also have a plan for people only want to use interpreters and not have them guide or plan their treatments (i.e, get a fee).
Using in house vs independent interpreters (nice summary)
You can usually scoop some up at airport kiosks when you exit baggage claim, but if you can't do this it's best to get in the city.
You can also pre-oder ones for pick-up at the airport here:
I have some information I wrote to a similar question here: