I think yours is a 10 x8. but check yourself ..if so an 11 x 8 should give a bit more grunt but pull more amps also.
http://www.ebay.co.uk/itm/APC-11-x-8-Thin-Electric-Propeller-Prop-E-LP11080E-/281767790315?hash=item419aab46eb:m:msKfgUB9o6sOCDW6cXgWSCw
edit : or check robotbirds...