Police in my city like to use wireless recording devices for their CIs. This isn't the 80s, ain't no recording devices they pull off of the informant later. They stick a wireless transmitter on him and listen to your conversation in real time in some location close by.
Is it possible to buy a jammer that just blocks all radio signals in an area? Does such a thing exist for civilians, even illegally through the darknet?
"Is it possible to buy a jammer that just blocks all radio signals in an area?"
I think you need to re-think your question. Radio isn't a single signal. It's a very wide spectrum. You would have to know which frequency LE uses. If you can figure that out, then what you need is a device that is close enough to the receiving device and that puts out enough data on the same frequency to overpower the transmitting device. That's the essence of a signal jammer.