Wal-mart even sells the oval lights and grommets now. You will have to get 4 (2 for brake lights, 2 for turn signal lights), unless you get a trailer adapter (as mentioned earlier) and splice it inbetween your existing wiring and the new lights.
"Trailer" lights are perfectly legal to use on vehicles as long as they are DOT approved (which I think they all are.....otherwise they will say something like "for offroad use only").