Why don't you put a shout out in here and in the General section asking if anyone could give you their small buds if they don't use them and offer to pay the P&P? I wouldn't have thought many people use the small ones.
Do they do anything like that on ebay so maybe you'd just pay the postage or even just couple of quid for them. Ooh..quick google and ebay has come up trumps - black earbuds 99p, you could probably contact the seller to see if they have the white if that's what you need.
Link here. Another set available at
Amazon for £5.
I'd happily send you mine if I wasn't using them myself. I tend to alternate between the medium sized and the small ones depending how securely I need them to fit.