It's in the beginning of Episode 124, when the Inner Soldiers are preparing to face the Death Busters at Mugen and they begin their transformations. 

Here's the clip of it:
https://www.youtube.com/watch?v=z5-f44WwZdg

I'm just wondering what the name of the BGM/OST is and where I can find it. Thanks!

I think someone else was looking for this song- let me see if I can find the title for you

march of the sailor soldiers

it can be found here