I searched for "LG Rumor" on eBay and found tons of accessories for this phone. Believe it or not, I got a Rumor-compatible USB cord plus a car charger (brand new) for only $1 + shipping ($4.50). Both worked perfectly with the Rumor. It even came with a CD that had tons of ringtones on it.
Unfortunately, there is very little documentation on how to use the USB with the phone. I discovered that a PC recognizes the phone the same as it does a flash drive. No software or installation is required. You can drag and drop mp3's into a folder on your SD card called "md" (which I think is short for "media") The songs then appear when you select "Media Player" on your phone.
In case you were wondering, it does not recognize mp4's (videos) on the Rumor.
Although I have been able to upload mid's files to the media folder, I have not fiqured out a way to navigate to those files for the purpose of making them a ringtone. And, like I said, there is no documentation on this.
Oddly, I wrote to LG product support reqarding the USB cable (before I found it on eBay), and they don't even have one. How dumb is that?
But they're out there if you look. I recommend eBay.