If you want to build one yourself you can check out my DS18B20 Temperature Probe article at:http://nuewire.com/2011/06/ds18b20-temperature-probe/
As robtillaart pointed out you can buy them from the Arduino Direct store in China and just this week Adafruit (based in NYC) added them to their store as well:
Waterproof DS18B20 Digital temperature sensor + extras http://www.adafruit.com/products/381
As far as gotchas go I would test a few to see if the reaction time is OK for your needs, the DS18B20 tends to be slow to adjust to a change in temperature. I would also watch the cable insulation as Adadfruit points out:
While the sensor is good up to 125°C the cable is jacketed in PVC so we suggest keeping it under 100°C.