etechparts.com carries lot of harder to find parts such as flex cables, audio jacks, etc, the pricing is not bad.
When I replace the volume/power flex cable I always de-solder the + terminal from the battery and slide a piece of Kapton tape between the cable and the solder pad. In addition, I place pieces of Kapton tape on the logic board anywhere I'll be doing solder work.
Last, call me a Kapton tape freak, I put a little Kapton strip across the new flex cable where the power button is --- this way if you have to lift in and out the logic board, say experiencing LCD issues, it reduces the risk of tearing.
Kapton tape, the right heat settings on your soldering station, a de-solder braid, some 60/40 rosin core solder, flux and 94% isprophal alcohol are what I use when it comes to solder work on the iPods/iPhones. I also use a 30x jewelers loupe to check anything prior to soldering - de-soldering to ensure their are no contaminates that could cause arching.
But, once you get your technique down you'll easily be able to replace back-plates, etc.