Went through this exercise recently when a rear blinkie fell off and got caught in the chain just as I started out from a dead stop. Not only did it bend the RD but bent the removable derailleur hangar. I knew the LBSs in my area don't carry the specific Shimano RD I needed (10 speed extra wide range) but it only took about 5 minutes to find a new one on ebay. Had the hangar in 3 days and RD in 5, both at prices similar to what it would have cost at the LBS if they had one in stock. All you need to know is the make and model to do a search online. Much less effort than checking with each LBS in my area and hoping for the best.