You're looking for a "roof rack delete kit", and there is more involved than just "the plastics".
From the roof surface upwards:
1. Thin rubber sealing washers;
2. Formed steel brackets that the outermost plastic covers attach to;
3. Thick rubber seals;
4. Short bolts with integral washer head;
5. The actual plastic covers.
Why don't you try a little Google search on the FJC roof rack install/removal and I'm sure it'll bring up a ton of threads right back here. It's actually better than our own Search feature. There's plenty of How-To's listed.