Oh dear, I think you might live up to your screen name when you get this answer!
I doubt whether you'll find a specific 'World Cup Sevens Shirt' anywhere. All currently licensed Springbok's gear is listed on the website of their associate company in South Africa, Signet Rugby. The only white shirt listed is referred to as the Alternate "Away" Jersey. If you know someone in South Africa, you could get them to purchase it and send it to you. This way you'd avoid the ridiculous international carriage charges that this company imposes. The shirt is listed at R495.61 (which works out at about �43). The problem is that the carriage charge to the UK is R459 (approx �40)!
Anyway, if you want to see if it's what you're after, check this link:
http://www.signetrugby.co.za/?Task=Category&Category ID=23316&HeadingText=Springbok+%2D+Replica+Gear
Usually I don't like to be proved wrong but, here, I'm hoping that someone will be able to say that I've got it completely wrong and that the shirt you want is easily available elsewhere!
Chris