I was able to get a Siminn kit a few years back at the Elko electronics store in the international area at KEF. The shopping area is straight ahead when arriving; do it before going down to baggage claim and customs.

At the time I did just voice only, so I don't know how their data coverage/rate are. The kit was in Icelandic, but they do have English support on the phone. They can also switch your account to English so balance messages, etc., will be in English.