To be pedantic, the advice (for those in your situation) is currently to wear a 'face covering', rather than a (proper) 'face mask'. That means that things like scarves are seen as just as acceptable as purpose-made face masks. (They certainly are at my local GP's surgery, where all visitors are asked to wear 'face coverings', rather than 'face masks').
However, if you still want to get hold of a face mask, any pharmacy should be able to sell you one. (Our local pharmacy, on the same site as the GP's surgery, charges £7.99 for a washable one or £1.99 for a disposable one).