Radio Shack sells a little amplified-speaker box, $9 or something. It's very noisy, and should output a reasonable amount of noise via the headphone jack. I can't vouch for its statistics.
The more formal way to do this would be to bias a low-power 10-volt zener to maybe 1 mA current, and amplify that. Figure the zener will make roughly 300 nV/rootHz noise density, or about 40 microvolts RMS in the audio KHz range.
You can also make noise digitally, with a pseudo-random shift register. See AoE.
John