I'd perhaps drop it by another shop in town. If the shop told you it was bent, why didn't they fix(replace) it? Did you refuse the service?
I ask, because if you're planning on replacing it yourself. A rear derailleur isn't an easy thing to do. It's not overly complicated, but requires some patience. If you decide to do this yourself, I'd suggest
www.parktool.com they have an area that's a "HOW - TO" that will help you in this endeavor! I've edited this post and found this specific address:
http://www.parktool.com/repair_help/...tml#drivetrain
BUT, a second opinion from another shop couldn't hurt!
L8R
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