Amazon has the cheap aftermarket batteries for most devices that take LiIon. I think I paid about 7 bucks for a spare EP1 battery. If you mean REALLY old like mercury button batteries I don't know where you could get those any more.
I just buy zinc/air batteries and stack them if necessary. I always also google first to find if the camera in question can work with the fluctuating current of alkaline batteries, since they are readily available
I can confirm the chipped batteries being sold on eBay DO work on the Panasonic G2. I've ordered 2 and took a chance as they're labeled as being "chipped"..........they work and charge in the OEM charger just as an original battery does.
I can't speak for battery performance as I've yet to take them out for any period of time but having them as spares almost certainly means a full days shooting should be available to me.
This was the seller I bought my batts from (no affiliation to them other than I've purchased from them) eBay My World - accstation