If you have a 3DS or a friend with a 3DS, the music app can sample almost any song down to 8-bit using it's cool 8-bit sound effect option. It's really neat to hear current Nintendo songs being scaled down to 8-bit. One can record it from the 3DS to their PC using a male-to-male 3.5mm audio cable, one end in the 3DS headset jack, and the other in their PCs microphone jack, and a recording app to record it into a digital audio file to be saved and imported into CUSTOM.EXE.
It doesn't work well for all music, some music just wasn't meant to be heard in 8-bit. I have the Ocarina of Time sound track on my 3DS(pre-order of OoT 3D got me it), and changing select songs from OoT to 8-bit sounds wonderful, such as the Lost Woods theme sounds nice. Some Mario Galaxy songs sound nice too. You can also change around the tempo and add other effects to make the song sound more unique for your game or the part in your game. Say the player is in the forest and during a specific event in the forest, the music should speed up for a chase scene, the tempo of the song could be increased however much you need it, trial and error.
Another idea, if a 3DS is not an option, is to look into Tracker software and find simple sound bytes of 8-bit sounds, and use them in the tracker to make your own music. I'm sure there is a Tracker instrument set for 8-bit.
Here's a link to a GameBoy Color software to track music directly on GameBoy hardware:
http://www.littlesounddj.com/lsd/
The software itself can be bought for $2. It would be an ideal way to create authentic 8-bit music and sound bytes.
Here is an site on how to make chiptunes:
http://www.ageofaudio.com/en/micromusic/chiptunes-making/
Hope these resources help you.