I sincerely doubt if you will find any free ones. If you are affiliated with a university or an organization that works with data, they should have access to some paid services out there. Mostly you pay for what you use like on Microsoft Azure. Another option will be to register for some workshop and use the opportunity to run your simulations. You still have to pay for the workshop though. Sorry I cannot be of much help on this
To my knowledge, there is no freely accessible HPC.
You need to either pay for computing time or apply (for free) for resources allocation. The latter is like submitting a grant but for computer time. French institutions have their program for sharing computer time (edari), there must be the equivalent on the european scale, in the US, China ...