one from off a Fiat Cinquecento will fit the panda. you can get them from off Ebay.
as far as getting the old one off as daft as it sounds, try picking the lock. they only have about 3 pins in them so are quite easy. have a look how to pick a lock on youtube.
if you are in or near derby i am pretty sure i could get it off for you. lets just say that its not only on computer games that i have a lock pick skill of over 80
If you plan to drill the old lock, or any other destructive method, you need to keep any debris out of the fuel tank.
Grovel under the car and disconnect the filler pipe from the tank. Cover the tank hole, masking or duct tape will do for the short time it'll be off. Then route the filler pipe downwards. Once the cap is off, make sure the filler pipe is thoroughly cleaned before you reattach it to the tank and pour fuel down it.