In February I saw tons of those face masks on Wish, which is basically an online jumble sale of all sorts. £1 for 10, £5 for 100 etc. Some packs were even free, just pay shipping of about 3 quid. At that point I along with nearly everyone thought the virus would just be a flash in the pan and wouldn't be a problem outside of China.
I didn't bother buying any, regrettably. The experts have said they offer little in the way of protection anyway, while others say they're better than nothing.