The best that I have found so far is https://opensky-network.org/ It even offers antonymous access to data, but why not join? The REST API is at https://opensky-network.org/apidoc/rest.html
offers free for non-commercial use data. However, it requires you to set up alive feed of your own data, to share it with others.
There is also https://uk.flightaware.com/commercial/firehose/, which also requires payments
http://www.adsbhub.org/ is another that requires you to share your data to obtain access.
As mentioned above, I have a similar interest in maritime data (AIS). It looks like my solution to my question Seeking the cheapest possible AIS receiver costing $60 / Eur 55 / £50 will allow me to share data and, thus, access that of others.
A similar device (an ADS-B receiver) for aviation costs from about $18 / Eur 16.50 / £ 15, and that's just a quick glance; no doubt Ali Express & BangGood will be cheaper.
These look excellent !! They get good reviews here. In fact, even though I can get free data without any hardware., these are so cheap that I will get one, just to play around with it.
You get what you pay for, of course, but I live only 2 miles from a major river and a major airport, which should be good enough for proof of concept. Plus, of course, even a minimal data stream should be enough to show willing and allow you share the data of others with better rigs.
Once I figure out how to set up the receivers and sign up to the APIs, I will update my answers to all related questions.