What do you call a person who always has what you need?
I would just say somebody is ready for any occasion or well-stocked.
I think the simplest term is prepared.
to make yourself or someone else ready and able to deal with a future even
Boy scouts are always prepared and if this person just had 100s of items and was a woman you could go with Mary Poppins.
They are often called as lifesaver.
something or someone which provides help that is badly needed
They might be called a supplier or a ready supplier.
There is a military term called provisioner also.