The bottom of this page has a chart showing the sizes of different chains. There are 5 different sizes that have 1/2" pitch and .306 rollers. If you have a thick rear sprocket you probably can't use 410 and if you have clearance issues #41 and .420 maybe out too. I've heard of guys finding 10' rolls at Lowes. Tractor Supply Co. if you have one. Maybe Harbour freight. Otherwise a motorcycle shop but you'll pay more. .415 is also known as #42 and #42H (heavy)
Azusa Engineering, Inc. Online Catalog - Roller Chain, Motorcycles, etc.