americium, for the uninformed, is present in cheap, non-optical type smoke detectors. the little grain of radiactive crap is inside a little lead case and has a hole in one direction, and a sensor goes directly in front of it, similar to a geiger counter, and when there is smoke in the air, the average rate of emission is reduced and it triggers the alarm.

probably you could adapt the radio-smoke-sensors to just feed a trigger that is read by your CSPRNG to reseed it, would just need a simple USB adapter that takes the signal and then sends some utility USB protocol message.