Don't apologize for asking questions, that's why we're here, man!
The fiberfill is most easily found in a crafts store or fabric store, though I imagine a hardware store may have something along those lines....
The fill should come in a large "sheet". Apply a little glue to the back of the box, and press the first layer in pretty good. Than just fold the material in untill the enclosure is about 2/3 full. It's not an exact science, so don'y worry too much. Even if the box is pretty full of the stuff, it won't hurt the speaker.