S&W packs a cable style lock with every gun. If you did not receive one go back to the gun shop where you bought it and ask where it is. I also think that all firearms manufactures are mandated to include a lock with each firearm.
Also, any store that carries firearms supplies/accessories, will carry trigger locks. Just bear in mind that cable locks and trigger locks are EASILY defeated, so do not rely on them to prevent unauthorized access.
I'm thinking of getting a little single gun safe