buy it or do some research on how to find booty
cant really help more than that on the forums
either way if its hard to find free with just google they probably don't want you using it and even if you were to buy it you're still breaking copyright by making a map unless you get permission from the artist. (not that thats ever stopped any other mappers ofc)
idk much about bms though, maybe someone can point to where to get those files and how to get audio out of them if its not just an audio file in a folder.