Version 6.3.15

This quest is for both factions but the starts are factional. You have some. The starter is a letter Elegant Letter item=17126

For Alliance this can be given by

Erion Shadewhisper (4214) Darnassus 34.43,26.05
Anishar (4215) Darnassus 37.91,20.79
Syurna (4163) Darnassus 37.01,21.78
Osborne the Night Man (918) Stormwind 74.64,52.80
Master Mathias Shaw (332) Stormwind 75.76,59.83
Fenthwick (5167) Ironforge 51.52,15.33
Ormyr Flinteye (5166) Ironforge 52.88,15.02
Hulfdan Blackbeard (5165) Ironforge 51.93,14.83

For Horde it is a bit problematic as I not long have an eligible character. From Wowhead etc comments it seems that they are obtained by all/some of the main city rogue trainers. I have seen video of

Ormok (3328) Orgrimmar 43.95,54.55
Mennet Carkad [6467] Undercity 83.50,69.09
Carolyn Ward (4582) Undercity 83.88,72.06
Miles Dexter (4583) Undercity 1458:85.20,71.57
There may well be more at Orgrimmar, judging by the apparent pattern.

I have no idea if there is a Silvermoon starter.

NOTE: As these quests have no '!', it may be easy for some to miss.