I agree with hosspuller that the John Deere website is best. After you use it a few times, it's not too hard to navigate. When you put in your model number you are presented with a list of tractors/implements that have that same model number. You have to select one you need parts for. After that, I choose the "sectional index".
I look up parts, add them to my shopping cart, then go to my cart to see the prices. That way I can do my cursing, wailing, gnashing of teeth, rending of garments at home, and then go peacefully to the dealership to get the parts. I have found that the parts guy at my dealership still has better information than the JD website and he has been very helpful and saved me from getting the wrong or unnecessary parts several times.
For maintenance parts (filters, belts, blades) you can find 3rd party sellers on the Web. I look up the JD part numbers on the John Deere website, and then Google the part number to see if there is a 3rd party seller. Sometimes you can even find discontinued genuine parts on eBay.