NiceBettas.com will sometimes have STUNNING mustard gas bettas. Keep an eye out- the breeder is a real sweetheart, too.
As for the black halfmoon, it's rare to see a true superblack full halfmoon betta fish without any blue or turqouise iridescence. Black Melano female bettas are generally sterile, so breeders cross them with steel blue or copper females. Even my superblack halfmoon plakat, Storm, has a blue iridescence. Black lace bettas have even more iridescence, and have transulcent ends on their fins, giving them the "lace" look. For the most part black lace bettas aren't even categorized into the black category, due to the amount of color they generally have. You can sometimes find black halfmoons on Aquabid, between the price of the fish and shipping costs you're looking at around $60.