They do have a PDF on their site: http://marketing.idquantique.com/acton/attachment/11868/f-00...
Basically, emit photons at a semi-transparent mirror. 50% go straight, 50% reflect 90 degrees. Have single-photon detectors at both directions. Quoting from their PDF:
Figure 2 shows the block diagram of the Quantis random number generator. It consists of three subsystems. The first one is the core of the generator and contains the optical elements that are used to implement the random process and produce the random outcomes. It comprises a light emitting diode producing the photons, a transmission element, where the random process takes place, and two single-photon detectors – detectors with single-photon resolution – to record the outcomes. The optical subsystem is controlled by a synchronization and acquisition electronic circuit. This subsystem comprises a clock and triggering electronics for the photon source, as well the acquisition electronics for the single-photon detectors. The processing and interfacing subsystem perform statistical and hardware checks, as well as unbiasing of the sequence.
Edit: the ID Quantique hardware is a bit pricey for hobbyists, starting at about 1000 euros.
If I were building something for myself, I'd look here for inspiration: https://www.fourmilab.ch/hotbits/ That hardware detects beta decay events.
That's preferable to me than "avalanche noise". Not that there's anything fundamentally wrong with thermal noise or shot noise; it's that in real implementations the quality of results is very dependent on things such as quality of power supply. To me it's much simpler to correctly detect intervals between consecutive radioactive decay events.
The downside to a cheap decay detector is that the number of events is much lower than what the commercial hardware provides. A few hundred bits per second versus a few million bits per second.