They do make 'plunge' bras which work well with deeply cut necklines. I have a few that I found at Victoria's Secret, but you may be able to find them at places like Nordstroms, Dillard's or Macy's. There is also a style called 'demi cup' that may work, in addition the the stores already mentioned, you may be able to find those at Walmart or Target. Although you won't be able to try them on first, Amazon would probably have all of those styles.
Love always -- Jess