AutoAnything is a great place to pick up accessories. Best part is if you PM [email protected], give him the part number you are looking for and he will come back with a quote with a RamForumZ discount.
Start scouring the forums here. That's the best I can tell you. You know what kind of mod's you want to do? There are forums here for the motor, interior and exterior lighting amongst other things like nerf bars, covers, ....... Maybe HID headlights? Oil catch can? CAI? Exhaust??
If you know what you want to do finding the correct source is pretty easy. Or maybe a pointer about a not so good source.
There are lots of sites out there, really depends on what you are looking to do and how fat you plan to take the build. AutoAnything is a great starting place for the basics-moderate modifications and where I got started with my build.