Not sure when it became available but by the time i finished the main missions i had the decker's sword and a chainsaw available at the weapons shop. There is also a cheat code that will give it to you.
I think it becomes available after you take the decker's stronghold. Either that, or after the deckers.die mission (or whatever it's called specifically). --- "I don't get chicks, man- if I came home and found some chick in my shower, that would be the best day of my life!" - Michael Kelso, That '70s Show
Wreck the deckers and then buy it from a friendly fire shop. --- TOU's vague, everything's a violation via catch-all phrases. Mods try to keep the peace but sometimes get it wrong. Post what you like, you'll be zapped anyway.