The source local to me is Jerry's Building Materials. They have rattle cans and, I think, quarts and gallons. For the fancy top coats dipyourcar.com.
Their tutorials are on Youtube.
A gallon ought cover the rear addition, but I don't know. They say 2,3 or 4 gallons to do a car. You could do it piecemeal but shouldn't the end goal be to have it all one color, even when it's cranked up?
Edit: I brushed a bicycle frame once and it wasn't successful. Maybe if I had thinned it but it had whiskers all over it. I think that was the dipping kind. Plasti Dip S is the sprayable formulation (it has more toners). A roller might be a good option.