We live in the future! If you have a wi-fi capable iPod, you can transfer files without your USB cable. I realized this the day after I bought the Nashville soundtrack on iTunes, and left my USB cable at the office. (This should have occurred to me, I download via wi-fi all the time!) If this happens to you, here's how to retrieve your iTunes purchases with the iTunes app and your wi-fi connection.
Turn on your iPod. If your wi-fi is not turned on, or you're not connected to a network, do that.
Launch iTunes via the iTunes app on your iPod.
Go to PURCHASED. It is the bar of icons at the bottom of your touchscreen.
Click MUSIC, the top bar on your touchscreen. It is followed by MOVIES and TV SHOWS. (These instructions will work with other media too. If you bought an episode of Justified, and wanted to download that, and not the Nashville soundtrack, then by all means, click TV SHOWS.)
You have some options. You can click RECENT PURCHASES if you just bought something.
The default listing is ALL.
I have 234 song purchases (this seems obscene, but the bulk of it is free playlists from hipster boutiques, thankfully), so the alternatives are faster. I know Old People are savvy, and expect your list is probably quite lengthy as well.
Another option is NOT ON THIS IPOD. Since your purchase is obviously not on your iPod, this is a very good choice.
Find the desired purchase. Click DOWNLOAD ALL in the top right corner.
You'll see a small red number at the bottom right corner on your touch screen, where it says DOWNLOADS.
You can press DOWNLOADS if you want to watch the transfer.
When the download ends, you'll find the music (TV show, or movie) in your iPod.