I would like to get a t shirt signed for charity?

hi i wanna get a teenage cancer trust tshirt signed for charity to auction on ebay, i know there were some sonisphere secret sessions in aid of it but does anyone know where i could get it signed soon and by someone reasonably big, eg. bfmv or biffy clyro . would u reccomned just asking them if i can post it to them by email or attend a signing...