At the German events the standard is paintball bottle. They have their own filling station.

For the Dutch events, that is covered.

I didn't see any filling station for paintball bottles at the RR events.

So, it's up to you to make sure you can refill your own bottle. It's also handy at home. You don't want to run to the paintball shop every time your bottle is empty. Easier to rent a bigger CO2 bottle and refill your own for testing purposes.

On low pressure buffertanks.
I have seen a lot of different things. But powder fire extinguishers are known low pressure buffers.