Kelloggs cereal boxes. I bought a few boxes of Frosties and stocked up on the vouchers, it's vital you read the terms though as the one's on one of my vouchers has extra dates that said voucher isn't valid for use (mainly summer at Thorpe Park, rest are peak times like halloween) that the other didn't.
But yeah, Kelloggs cereal boxes are where I got mine for the year.