There isn't one really for the amateur TRNG builder. Have a look at the frequency allocation for 0 - 300 GHz below:-
There are no gaps. A common First World problem is the provision of spectrum for all the intended uses. That's why microwave ovens share WiFi spectrum and Freeview TV competes with mobile phones.
What might seem a quiet (or hissy) gap between radio stations just means that you've not picked up anything at that time. Wait till sunset when the Ionosphere is on line, and you might hear France. Similarly when Stan fires up his arc welder next door you might hear that, or a passing police car.
The other significant issue with unpredictable entropy sources is measuring the entropy rate. That's notoriously difficult for a simple fixed source, but much worse if it can vary in frequency and /or amplitude. And radio varies so much, notwithstanding atmospherics. A mistake in assessing the input rate to entropy extraction risks breaking the golden rule of TRNGs (entropy out < entropy in). So there is the risk your DIY TRNG becoming a common pseudo random number generator.
No one uses radio noise as a TRNG for these reasons. I specifically and with prejudice exclude random.org. That's a private organisation with very little hard detail of how they do what they claim to do. And clearly they only have access to exactly the same spectrum as everyone else.
The hiss is actually thermal noise on the radio's detector and pre amp circuits. You can short circuit this and directly exploit thermal /avalanche noise in a reasonably simple custom circuit pulled off the internet. You can get 1000's bps of entropy this way.