Oddly enough a shirt company is making a Lucas shirt It's called fangamer. It's not out yet but they are making it.
You say that like Fangamer isn't run by the same people behind Starmen.net
Also, I don't think there is a specific number of stripes. There are few stripes on his shirt in M3 because it is tiny pixel art that had to fit on the GBA's small screen. And Lucas' (and Ness') character design in SSB is almost as if he is a midget -- very short body. A real shirt with that few stripes would result in very large stripes, which may or may not be your preference.
But yeah, no need to be exact because I don't think either number is "correct". If you can find any T-shirt with red and yellow stripes and no pockets or logos on it, I would snatch it up! I never tried looking, but I imagine they are just as hard to find as a Ness T-shirt was before Fangamer released theirs.