I bought one at the local auto parts store for $20. It works great. I was able to narrow down my rod knock to the crankshaft with it. The knock sounded like it could be coming from anywhere. I was able to put it in different places and hear what was going on - could hear the rockers working, the water pump impeller spinning, electrons moving through the alternator (not really), but the knock really got loud when I put it on the timing cover right by the crank.
A $2 piece of 5/8" heater hose works well, too.
Jim