it will eventually appear in the stores when you arrive on the southern island. it costs around $3,400 ish. you'll get for free minus attchments and skins when you unlock a certain number of radio towers. i personally just got to the second island and you unlock the deagle and vector for the first southern tower you activate, assuming you already unlocked all the towers in the northern island
Update: you need 15 radio towers to be unlocked and it will be your fore free, as well as all other weapons that can be unlocked this way. only weapon the needs more than 15 towers completed is the signature weapon called the bushmaster, which requires all 18 towers. if anyone would like to talk via mic im on quite regularly and in dire need of xbox friends to at least talk with......very lonely.