You didn't mention if this is a SOHC or DOHC bike. I'm not sure if it matters or not, but if it's a DOHC model you're asking about, here's a link to some for a decent price.
http://www.motosport.com/cruiser/oem-parts/HONDA/1981/CB750/STARTING-MOTOR/SPRING,-STARTER-CLUTCH-ROLLER/28125-516-030A Google search for the part number should be fairly easy, once you get the official Honda P/N for the spring. Bikebandit.com should be able to help.
EBay is your friend as well.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.