Hi Rookie and welcome to the Q&A Board!
I use generic batteries for my Nikon and they work fine. I didn't buy them on ebay but Sterlingtek, which many Nikon digital camera owners use. Not sure they have them for the FZ5 but you can check it out.
http://www.sterlingtek.com/polcamacpag.htmlI've read of others who bought a cheap battery on ebay and were satifisfied. I think the Panasonic proprietary batteries are expensive, so it may be worth a try to buy one inexpsensive, no-name brand battery and see how it works out. Another option is to do a
Web Google Search and see if you can but a Panasonic brand battery at a discount.