I use the KTM Hardparts 40 tooth sprocket, it's made by Supersprox so you might be able to get it cheaper directly from them. Also mine is a 2008 so it came stock with the 16 tooth front sprocket. Response is fantastic.
Happily only have 4200 miles on it and haven't needed a chain yet. Just keep on cleaning it and lubing it and hoping to get the most out of it I can.