I don't recall who did it, but someone once pointed a webcam at a row of several lava lamps as a source of entropy. As long as at least 75% of them were on at any given time, the image-hashing function produced unpredictably random numbers. The low quality of the camera produced some random noise all by itself.

I imagine you could do something similar with a feed of traffic on a nearby highway or blobs of cornstarch suspension on a subwoofer.