Temperature probe + screen that is accurate reading sub-zero temperatures

I've currently got a Commercial 3.5" bay-drive temperature readout device that has two-probes, and a little LCD screen that shows the temperatures being read. However, through primitive testing I've discovered that the device doesn't read sub-zero temperatures very accurately at all.

I'm wondering if anyone could reccomend a good commercial temperature probe/screen that accurately reads out sub-zero temperatures onto a small LCD screen? I only need something accurate down to about -20 degrees celcius, if that.

Ideally, I'd want something small, that could be powered by my computer's PSU. I don't need something that mounts in a bay drive.

I appreciate any links and advice

Not sur eif it's 12V or not, but radio shack has some little temp sensors. Seem to be accurate within a degree or two down to about -30C or so.