Check http://www.findchips.com This shows 6 distributors that stock the 18B20.
Where are you located? I'll send you a used one (at least we know it's working)
if you spring for the postage.

But, before I wrote it off as bad, I would check with what KVLV just said. You might
try using a different I/O-pin, different oscillator speed, etc.

Try a timing test with say an LED on a port pin. Toggle it on & off. Does it toggle at the
speed you expect? If not, it could be a timing issue, and that would definitely be a problem
with 1-wire comms.

LCDOUT can sometimes work just fine. Even if you have the wrong oscillator speed declared.
So I would run a series of tests before I wrote the sensor off as bad. Just in case.