Harken (HSB56) and West Marine (202523) both sell the replacement springs. If you've busted a spring, chances are your cleat's pretty old...I actually managed to wear one out. Take a good look at the teeth. If they're really worn, it may just be time for a replacement...also available from Harken (150) or West (114652).

Jerome Vaughan
Hobie 16 "RATTLE 'N HUM"
Clinton, Mississippi