The WEP key is available on your router.
In any browser, type in the web address "http://192.168.1.1". That is the address of your router on the network.
Your router will have a user name and password...check with your router's manufacturer to find the defaults if you haven't changed them.
In the wireless security portion of your routers settings, the WEP keys will be visible. Write down the first key and use that with you PS3.