It's been quite some time since I played the game, but if I recall rightly, you're given the flamethrower before the Hornet's Nest mission. I'm also pretty sure you can find one in some of the small structures near the marijuana fields.
Can anyone confirm this? Like I said, it's been quite a while since I played.
You need to take down 6 radio towers to have the weapon unlocked. I remembered going into the weed burning mission with an upgraded flamethrower, so you can definitely get it before that mission.
Best weapon in the game, amirite?. I remember killing a helicopter with this bad boy.. Ammo is an issue for long hauls though.