Edge of Cosmos + Latro’s Shifting Sword would be your best bet in terms of easy to acquire.
Latro’s Shifting Sword in particular is simply your Phase 1 BiS, there is no better Offhand Sword from Kara, Gruul and Magtheridon.
Vindicators Brand isnt hard to acquire either, it just takes some time to farm enough Marks/Armaments and to do the few quests.
Alternatively you can opt for a Fist Weapon as Mainhand and go with Demonblood Eviscerator from Shattered Halls/The Bladefist from Shattered Halls Heroic. However you would want to Respec and get 5 Points in BOTH Sword Specialization and Fist Weapon Specialization then.
Blinkstrike/Dragonmaw if you got the gold
Vindicator’s brand if you don’t (can easily farm it in like a day)
Bladefist - If you got the balls to farm SHH.
Offhand Latros from BM, no other consideration needed.