You could always pull out the other one and take it to your local hardware...but it'll probabably drop on the floor and you'll lose it
Besides, from my experience, they're not going to know the diff between Whitworth, BSB etc. They should, but they generally don't. They'll sell you a wood screw. They come in packs of 20s at $$$$s.
EB Customer Service could probably do you a great deal \