kerrat (very low chance), stf optional lootbag (even lower chance)...or exchange, if you got the financial background to afford them.
But they are not Fleet Weapons.
The OP asked what mods are best and although I agree with you for the ACC the ACCx3 are not a fleet weapon mod.
All the fleet weapons are Dmgx3 + 1 other mod, none of which are Accx3.
They are the bee's knees with just only 1 point between the Fleet cannons and the Quads and given the quads actually give you a engine negative one could argue the Fleet weapons are better overall although I ain't going there cause I love my quads lol.
*sighs* The spillover from ACC is NOT as much as a dedicated CritD modifier. Also, spillover doesn't NOT convert to additional CritH chance, it only converts to CritD damage, afaik. Anyone that says otherwise, please show me documentation.
There seems to be a lot of misconceptions about the need to run ACCx3 modifiers non-stop. In PvP, yes, acc is important, as many players are running with the Elusive trait and in an escort. But don't ill yourself with ONLY finding accx3 weapons.