Airlocks cost about .75 each. The rubber cork w/hole (its referred to as a Universal Bung in my wine catalog) that comes with it is also about .75.
Since the top of an airlock is capped, it might be possible to use two airlocks, remove the caps, silicone the tops together, and then the other two ends could be pushed into the tubing. Maybe I'll go down into the basement today and try. I'll let you know if it works.
Also, be aware that there are two types of airlocks...the squiggly test-tubish paper clip one that Digger described so well, and a new fangled version which doesn't have any bends in it...its like a tube with an inverted cup inside it.
I'm not very good at describing these things...I'll try and find a photo if I can.