I will arrive in Iceland at Keflavík International Airport at 23:10, and would like to take a Stræto bus within Reykjavík the following morning at around 07:30. How might I get hold of the ticket price of 440 ISK in coins? Advance tickets are sold in packs of 20 only, and on the bus tickets are exact change only (see Stræto website). The smallest bill is 500 ISK but an ATM might not dispense anything smaller than 1000 ISK. Can I exchange bills for coins at night anywhere at Keflavík Airport or in Reykjavík? Alternately, can I purchase Icelandic coins at currency exchange in Keflavík, so late in the evening? I think currency exchange usually do bills only, in particular in case I would try to get hold of Icelandic currency before the trip.
For future reference: You can buy single tickets on the Strætó mobile app now. Also, the following shops/kiosks and terminals sell bus tickets and bus cards (which includes a list of the 10/11 convenience stores mentioned in the comments above).