find a hardware store with 1 1/8" ID tubing, get ~4 ft, sanitize it and stick it into the opening of the carboy. other end goes into a small bucket of sanitizing solution like iodophor or starsan.
alternatively, you could fit a 1/2" ID piece of tubing over the center tube in the airlock and run that into a bucket of sanitizing solution too. do it soon or the top will pop.
edit: i'm sure there are a few sizes of tubing you could fit over the airlock tube, try some that you might have laying around