You may have to get a commission done. However I think mascot costumes are pretty pricey. Maybe you could get some of the costume commissioned and try to complete it yourself..? Or maybe there are some mascot patterns out there, but I'm not sure.
A-kon 2009: Miku Hatsune (Vocaloid) 100%, Kantarou Ichinomiya (Tactics) 100%, Yuki Nagato (Haruhi) 100%
A-kon 2009 Masquerade: Miku Hatsune (original princess design) 100%
A-kon 2010: Simon (Gurren Lagann) 40%, Haruhi (Host Club) 10%
I do commissions for wig styling, props, jewelry, and weapons!