Any repair shop should be able to help you. Just tell them your problem and have them check it over and give you an estimate. I wouldn't consider replacing anything until the steering and suspension are checked out. The issue with dealers is that they can be very expensive. For a site that will let you check price estimates:
RepairPrice Estimator - Get an Estimate for a Service or Repair on Your Car
As far as parts, it depends what it is. When it comes to ignition parts, starters and alternators, it's best to stick with genuine Nissan or NGK. For basic mechanical parts, ie brakes, shocks, etc., it's okay to go aftermarket, but use a reputable brand, like TRW, Beck-Arnley, Hitachi, etc.
For genuine Nissan parts:
1st AAA Nissan Parts Store
For aftermarket parts:
RockAuto Auto Parts