Location How to get Refined Iron Sand in Black Myth Wukong

During Act 2, when you're up against the First Son of Flowing Sands, there’s a large breakable wall in the area. If the boss smashes into it or you hit it with a powerful attack (I used the Wandering Wight Spirit Skill from Act 1), the wall will shatter, revealing a Loong Scale. This scale gives you access to secret bosses in hidden locations, like the waterfall in Act 1 and the sand wall in Act 2.

You won’t get any refined iron sand from the secret boss in Act 1, but you can challenge him by using the Loong Scale at the waterfall near the Outside the Forest shrine.

After obtaining the Loong Scale, head over to Rockrest Flats, where you can now break through the sand wall. Once you defeat the Black Loong boss inside, you'll be rewarded with 2 refined iron sand and a Thunderbone weapon material.
 
Dont know which point you're at the in the game but you can easily get 4 Refined Iron Sand in Chapter 2
 
You can also get Refined Iron Sand before Act 3 by locating the Drunk Boar in Rockrest Flat.

If you give him a Sobering Stone (which can be bought from the Man-In-Stone or found in Windrest Hamlet), the boar will move to Crouching Tiger Temple.

Once he's there, speak to him and bring him a Jade Lotus. Afterward, return to where you first encountered the boar.
You'll have to fight him, and once you go through the crawlspace, you'll be transported to a hidden area called the Kingdom of Sahali.

The bosses in Sahali don't drop Refined Sand but the store here sells 1 so I hope it helps you out man