Just a few weeks ago I got interested in this as well ("where does FR24 get its data from?"). I ended up buying a cheap RTL-SDR dongle (R820T2) and a small outdoor antenna. I run the free dump1090 tool (I'm on Fedora) to decode ADS-B messages, then my own simple "radar-like" visualization program ([1]) connects to dump1090's network socket to receive decoded data (SBS1 textual format). Even with the antenna just sitting in my room (on a photo tripod), I typically receive data from 10-20 aircraft, up to 190 km away. I drove to a hilltop this weekend (some 600 m higher) and immediately got >100 aircraft, up to 500 km.
[1] https://github.com/GreatAttractor/plane-tracker