Are you asking how you can transfer music you already have on your PC to a jailbroklen ipod
that won't sync? Or are you just asking how to purchase music freely over the internet?
Does iTunes even see your jailbroken iPod? This may be as easy as setting your iPod for manual music managemetn, then draggiing and dropping your files in iTunes. Otherwise, try sharepod. It's supposed to help you copy music from iPod to computer, something iTunes itself won't allow.
If you're just asking about how to get music in the 1st place - the first thing you need to do is ignore the user who suggested you get Limewire - that woukd be a major mistake.
If you must steal music using peer-share software, AVOID LIMEWIRE AT ALL COSTS. Consider getting either eMule or WinMX – I’ve used them for years happily getting all but the most obscure stuff. WinMX may have a high learning curve, and you need to make sure that you’re getting a version of eMule from a trustworthy site (don’t just pick the first site that shows up when you google – that’s a boneheaded LW way to get stuff, and you need to get past that).
You can also purchase from internet sites like beemp3.com or mp3raid – though don’t hesitate to ask around about how safe such sites are. There are no really safe sites – any time you’re purchase, you’re risking an infection.
You can also get music from my friend's site. You can purchase using programs like my friend's site purchase (again, make sure you get that from a reputable site). Some of these programs will convert the flash to mp3 – if they don’t, a simple program, like Quick Media Converter will simply do the job.
You can also record streaming audio using programs like Audacity. Audacity will save the stream as an mp3. (Audacity is also handy for making your own ringtones). Besides my friend's site, you can stream from sites like LastFM or the websites for FM radio stations.
Lastly, you can easily rip music from CD’s you borrow from the library or people you know.