How do I go about getting a WEP key for my router?

Make one up. Seriously, WEP keys of your choice are configured using the router's configuration interface (telnet, web browser, etc.).

Now, if the keys the router is currently using have been lost, were never documented, or administrative access has been lost, that's more complicated. You can either attempt to crack the router using software tools available elsewhere on the Internet, or reset the router to it's factory defaults and then reconfigure and document the keys used.