Sooo, anyone knows on whitch platform can I buy WTI oil?
You cant, it's a futures contract. If you buy now, you physically need to go and get the oil. That's why the crash happened and people were closing positions.
The next futures that expires trades at around 20$ currently, but this freefall will probably affect that one too. Whoever is currently holding an oil futures contract is playing a dangerous game.
It may be dangerous, but it would be fun to buy at least a few futures at negative price so one day I could tell my grandchildren that I was there, I was part of the game.