Hi Andrea,
The problem with buying on-line is that unless you are sure of the vendor etc you can get fobbed off with fakes.
However, you can buy extra ones from your regular pharamacy, what you have to do is see your GP, who can write you a private perscription, I did it myself a few years ago, you have to buy in multiples of 8, the price comes down the more you buy, they used to be about £4 each if you bought a batch of 32, they may be cheaper now, I seem to recall that you pay the GP not the pharmacist.
Sad to say in recent years 4 a month has been enough for me, so best of luck, go for it, and consider it money well spent.
:)
Dave