A few weeks ago my S3 croaked on me

Symptoms: was riding home, electronics went dead, got home, bike shut off. Would not start. I charged the battery and bike starts right up again. While running, I disconnected the battery cables and the bike just dies. I suspect the alternator is at fault. This is my first Triumph, so I rather try and fix it then part with it. Where to buy cheap parts, the dealer wants a grip??? Any help or suggestions is greatly appreciated!!!
Hoping to ride again SOON!