How about a padlock or bike lock?
Seriously though - maybe some sort of metal clip? It's really hard to say without seeing what you're talking about. Depending on the situation, maybe one of those clips for keychains?
ROADPRO 80mm Aluminum Gear Clip with Keychain - Assorted Colors RPGC-80A
Or a leash snap?
NP SWIVEL LEASH SNAP by Baron Mfg. - More Harness & Chain Snaps, Links & Hooks at doitbest.com
Or a lock pin?
SPEECO LOCK PIN ASSEMBLY Secures PTO adapter to shaft P7940 SPECIAL SPEECO PRODUCTS
I'm sure if you made a trip to the hardware store you could find something.
