What can be used to replace a Shimano 105 5600 long-cage derailleur? I have a Fuji CCR3 with a triple in front, 9-speed casette in back.  It came with a Shimano 105 5600 derailleur, but no local shops stock those.  I've also been told that the 5600/5700 are 10-speed derailleurs and shouldn't be used on 9-speeds (though they can, they just don't work very smoothly).

So, what should I replace my (broken) derailleur with?
Depending on your gearing you might be able to use the fairly widely available RD-4500-GS.  I personally haven't seen a ton of difference between 105 and Tiagra components.  The sora derailleur is also available in a 9 speed configuration.