From what I have learned in the Chinese Buddhist tradition, there are many mantra and dharani that are in sutra. I understand that the use of them does not require transmission by a specific kind of teacher. This practice is still called esoteric, in the board sense that most human practitioners do not know how and why they work. Just not esoteric in the sense that they are secretly taught by a teacher with a lineage.
Having said that, some of these dharani and mantra practices as contained in sutra can still be specially taught by a teacher. Common examples are the Great Compassion Dharani, the Cundi mantra, certain preta-dana practices. One can simply practise them by learning from and following the sutra. One can also learn it from a teacher, along with hand seals (mudra I think they are called), visualisation/contemplation techniques and sometimes specific vinaya rules. Now some of these have been taught publicly by monks and lay practitioners in the past, with some even having made their way onto the Internet. If these monks and lay practitioners thought the practices could be taught to the public, whether you learn from them and practise accordingly is, I think, your choice.
Namas triya-dhvikānāṃ sarva tathāgatānām!