Tyler, it keeps the paint from flaking off of plastic parts, and should be added to the primer as well before you actually paint the part. You can get it from any store that sells auto paint.
Some paint and clearcoats has it already in it and some paint shops will recommend adding a little more to the paint.
Just go to a auto paint store or any store that sales auto paint and tell them what you are wanting to do, and ask about the flex additive.
Most part stores carry interior paint kits that work really well, IF you follow the instructions to the letter. hope that helps you out.