I got a replacement battery for my iriver H10 20GB from a site called iPods99.com. They have replacement batteries for virtually every model of iPod as well as for other models of mp3 players and other similar things...
That being said, the battery I got was a Cameron Sino brand, and it works beautifully. You can probably find them on eBay for relatively cheap, possibly cheaper than from that site.
Other people may have other recommendations, though... *shrugs*
EDIT: So far as voltages go, it should average 3.7 volts, I believe (I think that's pretty typical across the board for the current DAP targets, isn't it..?), and try to get one a high mAh, since the higher that number, the longer the battery will last on a charge.
Also, make sure the battery you get will fit your iPod. If you search for your iPod model (in this case, 30GB iPod Video) + battery, you should pull up decent results.