Have you considered getting the dropout checked and realigned (if necessary) by your LBS... here's the procedure: http://www.parktool.com/blog/repair-...lignment-ffg-2 , then buying a derailleur and putting it on yourself?
You'll need a chain machine and spare pin (Shimano) if you don't have a masterlink on the chain (SRAM).

Adjusting the derailleur is described here

Sorry if you know all this already

edit: the chain can be installed without breaking it by disassembling the rear derailleur, but this is maybe more hassle...