Where can I find the working or mantra of Brahmastra?

You can’t.

After the battle of Kurukshetra, which was a horrific battle, in fact it was a mass slaughter such as the world had never seen before, where all the rules of traditional Vedic warfare were broken, the brahmanas placed a curse on the Dhanur-veda (the Veda of warfare), to ensure it would not be functional in Kali-yuga.

The Vedic Version states that all creation evolves from sound. First there is sound and sound produces space or ether. It even says in the Bible - first there was the word. So all the elements of matter proceed from sound. It goes from subtle to gross - ahankara or false ego, intelligence, mind, air, fire, water and earth. These 8 elements and their qualities or senses constitute matter.

The sense of space is sound. The sense of air is feeling. The sense of fire is form. The sense of water is taste. The sense of earth is smell. Also, the quality of an element is present in the one that comes after it.

That's how the yogis and sages of former times had what we today would call supernatural powers. It's not supernatural, though. It's completely natural. You just have to know the science behind it. When you know the right sounds, you can create or dissolve matter. This is known to some degree in modern science after it was observed that certain sound frequencies can splinter glass. And the Bible describes how the walls of Jerico were crumbled by sound.

So, the Shruti portion of the Vedas contain mantras, knowing which, one can manipulate matter. Obviously, this only works when the mantra or the sound is pronounced exactly right. That's why you need to learn this from a living teacher. Shruti means that which is heard. You have to hear the mantra and learn how to pronounce it, in order for the sound of the mantra to have the desired effect. For instance, the Bhagavat Purana documents an event, where the brahmanas killed king Vena by uttering sounds.

Now, the shmriti portion of the Vedas contain the philosophy and theology, the moral and ethics, and the histories of ancient India. Smriti means that which is remembered. You can learn about philosophy and religion by hearing it from someone who has remembered it. Your understanding of what is being taught is not dependent on the the correct pronunciation of the Sanskrit. You simply have to remember it, and it will work even though you don't utter the words in a certain way.

That's also why the Sanskrit of the Vedas is different in the shruti and the shmriti.

Some scholars have suggested that it shows the Sanskrit has changed over time, but that is pure speculation without any support in the Vedic tradition. According to the Vedic tradition, what is seen is simply different types of Sanskrit meters in shruti and shmriti.

The shruti mantras are to be pronounced exactly precise, in order for them to work. The shmriti mantras just have to be remembered. Philosophy, theology, religion and history you remember. Mantras to kill people or to create strong winds and rain, or fire, or a nuclear blast like the brahmastra, have to be pronounced exactly correct. Kardama Muni created an entire mansion in the sky by sound vibrations. That happened in Satya-yuga, several million years ago when the science of uttering mantras were known to the Vedic sages.