I don't use a Mac (or AIFF files), but
I think the iTunes software can probably do it. And of course, iTunes can make AAC files which are standard for the iPod.
Max can probably do it too, but XLD is supposed to be for lossless formats, and MP3 is not lossless.
You shouldn't have any trouble finding software... MP3 is the most common, most popular, audio format. And, I think AIFF is standard for the Mac. So, I'd bet that any "MP3 encoder" software for the Mac will do it. There is a list of software that uses the LAME MP3 encoder on the
LAME website.
For Windows (and WAV files) I use
RazorLAME, and I've used
XRECODE a few times. I'm not sure if these will work with AIFF files, but if you have to convert to WAV first (with another program) , the conversion from AIFF to WAV is lossless... It's extra work, but there is no harm.
You could also make the conversion with an audio editor (open in one format, and save in another).
Audacity is FREE (open source), and there are Mac & Windows versions.