First off, try fender uk - they may have a set.
If not you could get the train up to Denmark St in London and try Rockers or one of the other shops, where you could buy 2 packs of strat knobs (Vol, Tone, Tone) leaving you with 3 spare knobs.
or fire up yer browser and go to:
[url="http://www.allparts.uk.com/plastic-knobs/for-strat-c-277_1_91_98_100.html"]http://www.allparts.uk.com/plastic-knobs/f..._91_98_100.html[/url]
...and buy a 2 pack of tone and a 2 pack of vol, leaving you with only one spare (tone) knob. Probably considerably cheaper, if you take into account the train fare.
And they're nice guys, so give them a call first to check that the knobs will fit the pot shaft.
You then 'age' the spare in a jar of cold tea (no milk) for a month and sell it on the bay as genuine 50's strat knob for £150.00