Samsung were carrying spare batteries when this first arose a while back - except that those turned out to be faulty as well! - but they've probably cut back after the problem was assumed to have been covered. There's been a recent spate of the issue recurring and I don't know whether it's a new batch of faulty batteries or ones from the original batch that have taken longer to fail. Battery stock in the retail channel is generally low, due to the lack of demand and the many variations. Attempts have been made, notably by Duracell, to standardise, but they've got nowhere.
Online sourcing is best and you simply have to pay what look slike a sensible price as it's not possible to tell what's genuine and what isn't. When this was around before, both Samsung and Vodafone were reimbursing people who'd bought their own. I don't know if this is still policy and I assume Vodafone (as a 3rd party) were indemnified by Samsung. Again, I don't know if that's still policy.
Although you have the right to insist that any fault is dealt with by Vodafone, if it's quicker to go direct to the manufacturer, that's normally the preferred solution.
