It is always advisable to take a mantra, containing bIjAksharAs like aim, klim, etc, from an accomplished Guru. An accomplished Guru will be knowing which mantra will suit to one aspirant. There are 2 phases in getting siddhi of a mantra. (1) Priliminary stage, where an accomplished Guru will study the nature of the aspirant and initiates one into appropriate mantra (2) Apart from reciting the mantra, one has to purify oneself internally, or soul purification. The more one becomes purified internally, the better results will be. It takes its own time.
The Bala and atibala mantras, which were mentioned in the question content by the OP, were composed by Sage Viswamitra and taught to Sri Rama alone. So initiation into great mantras require great purification of mind like that of Sri Rama. Unless one is qualified, no accomplished Guru will teach such great mantras to everyone in the street.
Even if learnt from somewhere else, the effect will be nothing.
Here is an anecdote on the importance of initiation.
A minister in a court was summoned to appear before the king. The
minister also happened to be a brahmin priest, a kulaguru, empowered
to give initiation in the Panchakshara mantra, 'Aum Namah Sivaya'. The
king asked the minister to initiate him and the kulaguru said that he
would, that he could start preparing the king, right away. The
maharajah was impatient and demanded that the minister initiate him
immediately, without any preparation.
Of course, the kulaguru objected and told the king that it was not
possible, as his tradition required him to only initiate after proper
training had been given. The kulaguru explained further that this was
a vow he had made to his own Guru, when he was empowered to give
initiation. The maharajah replied that he would do the mantra without
initiation, unless the kulaguru explained how the mantra worked
differently, without initiation. The kulaguru agreed to explain the
The guru then called loudly to the five guards, who were present and
told them to arrest the king, because he had demanded initiation
without undergoing the necessary preparations that his tradition
required. Of course, the guards did not act on his order but instead
looked toward the maharajah waiting to be told how to respond.
The king was quite upset at the preposterous order of his minister and
shouted to the guards to arrest the kulaguru. Of course, the
guards quickly came forward and grabbed the guru and held him tightly.
The kulaguru smiled and explained his actions by saying that what he
said was just answering the king's question. The guru went on to
explain that he had spoken the mantra, "Arrest this man".
But, as he had not been prepared for and coronated, and thus empowered
as king, the guards did not respond. However, when the Maharajah gave
the same mantra, "Arrest this man," because he had been properly
prepared and coronated as king, the guards responded. Upon hearing the
explanation the maharajah understood the importance of proper
preparation and initiation and prostrated at the kulaguru's feet.
If the OP wants to do mantra japa, without initiation, it is better to do pArAyana or recitation of Durga Sapthsathi, which contains the mantra with bIjAksharAs.
ॐ ऐं ह्रीं क्लीं चामुण्डायै विच्चे।
Unless initiated and done in a proper way, this mantra japa will result in nAma japa only and the Devi or Shakti will direct the reciter to a proper Guru for initiation.