Sounds like you didn't install plus properly, or you're trying to open it from JCS which opens vanilla JJ2 by default. Don't bother wasting time converting it to S3m, it's a waste of time and space.. If you don't use a tracker like the way it's meant to (reusing samples), you're basically using it like a WAV. So it becomes huge.

Installing plus isn't just about creating a Jazz2+.exe with plusifier. I had done that before when I was new to plus when I didn't know what I was exactly doing. What was the problem? I didn't paste all the plus file contents into the specified jazz2 folder. Ctrl + A all the plus file contents and paste into the jazz2 folder, *then* use plusifer.

However, jj2+ is naturally named jazz2+.exe. When you open Jazz2 from JCS, it opens jazz2.exe. To make JCS open plus, rename jazz2.exe to something like jazz2-.exe. Then you need to rename jazz2+.exe to jazz2.exe.

My best advice for finding music for jj2 levels however, is to look for videogame music or just take a lucky dip (aka just downloading random music) on free music sides like the mod archive.