There are a couple of guides here that should help. http://sheldonbrown.com/tooltips/bbadj.html
Before considering a replacement BB you should service the existing one. Sheldon Brown suggests doing away with the bearing cages and fitting all new balls but you should be fine just cleaning and lubricating what's there. Pop the balls out of the cages so you can clean both properly. I'd be surprised if that doesn't bring your BB back to life. If the cages have broken up you can either buy replacements or buy new balls and put it together without cages as Sheldon Brown suggests.
1979 Dawes Fox
1982 Dawes Super Galaxy