I live in Nha Trang a beach city in Vietnam. We have a couple of good biikini shops here, sorry I don't know the names but one is on Tran Quan Khai next to Jungle Travel. As well as a pretty good selection they will also make them for you in 1-2 days, choice of fabrics and styles, or they can copy an old one you have. They range between 300-500,000 vnd [$15-25]. The other shop on Hung Vuong, after Rainbow Divers does the same, they have a few Victorias Secret
bikinis, a bit more expensive there, 500,000.