@WorldMap, FWIW, recently I came across this data provider:
In addition to some stock exchanges they provide access to many crypto exchanges data (RT, historical almost at any interval).
The crypto data is mainly accessible via API (costs increase based on the frequency of the API requests per minute) and Websockets.
One thing that is not very common is the use of Unix Timestamps for all quotes.
I imagine that via a script the historical data (in Json format) can be converted and brought into Amibroker.
For those with programming experience they could be one of the providers to query RT with the plugin that @nsm51 is developing.
Unfortunately I have no idea about the quality of the data, nor the reputation and reliability of the company (I don't know anyone who uses it - I recommend doing some research before possibly signing up for trial subscriptions).