Audacity
MMD can only use .wav files for audio, so you cannot use MP3's and such. Well, what if you have a song you really want to use, but it's not the right file type? I've got the answer for you! Audacity is a free audio program that allows you to edit and convert music files into whatever you want. Just load the song into the program, go to File>Export Audio, and change the file type to .wav, then it'll ask if you want to change any information on the file. The point is that you can easily convert a song into a .wav file using Audacity.
Zamzar File Converter
Alright, so you don't want to download a program. I get you. It's totally fine, because we have another option, this time it's all online with no need to download a program!
This is a website where you can upload the song you want and convert it into a .wav file that is compatible with MMD. I have converted tons of songs using this website because it's fast and easy! It doesn't alter the audio through the conversion, it sounds perfect. Just upload the file, give them your email and once it's converted, you go to the link in the email they send you and download it from there.
YouTube to MP3
Now what if you want an audio file you cannot find, and it's in a video without a download link? Just go to the YouTube to MP3 converter to rip the song from the video and then send it to Zamzar Online File Converter. This process is very easy and fast.
NOTE: I am not telling you to go to videos and rip audio from them to re-upload and use for profit or such, just use them for MMD, please.