I doubt if anybody ever made an 8 speed freewheel with a 11t cog. There was a Shimano 7 speed freewheel made in 11-28t and 11-34t that seems to be impossible to get now? For 8 speed freewheels look at
http://sheldonbrown.com/harris/freewheels.html#8 . Are you sure it isn't a freehub w/cassette? They sell a 11-30t and 11-32t 8 speed cassette. Worst case you can buy a new wheel w/ 8 or 9 speed freehub for $60 (and up) and add the cassette of your choice. You might have to also change your shifter and/or derailer if the spacing is different from what you have now?