How to pay Canada from overseas?

I'm in the UK and trying to unlock an iPhone that is locked to a Canadian network, the only problem is paying with my UK debit card.
They say they can't take it because they can't verify with a Canadian bank that its not a case of fraud.
They suggested a pre paid visa card might work.
Any suggestions on which pre paid card to use for it to work?

They can take it, thats rubbish, if it is a Visa debit card, the Canadian bank that they are using to clear the card will verify through the Visa or mastercard network, that the card is genuine, it will verify the card number, epiry date, CVV number at the end, your name and also your UK address and everything will be fine. Go back and tell them this information. Visa and mastercard is a global payment system, so what is their problem!