Honestly they're all over but here is what has worked for me easily. In the Divide there is the room that spawns shanks which go back behind the building where the drop ships drop enemies. Go there and every 10 minutes a group spawns. For me it was pretty much on the hour and every 10 minutes after. You can kill them quickly. The downside is that you of course have to take about 30 minutes to do it. I however took that time to finish other bounties in the area around there like knuckleduster, etc.
I *think* the whole planet spawns swords at the same time.
I went to steppes (underground next to the three yellow bar hive) then to mothyards (warsat/near the wing ramp to the 'secret' cave) and got two swords in a row. then tried to find one at forgotten coast but couldn't find any. I saw pikes raoming so maybe that happened instead?
sidenote, rocketyard spawns the sword in the middle where fallen and hive fight, or the far corner near the entrace to a place that isn't implemented yet, replacing the fallen group there.
if you did no story missions, the first mission has a sword spawn right inside the first building you enter. you can load up the mission, sparrow past the enemies you spawn into, get the sword, go to orbit, repeat. but I don't think you can do that mission again if you completed it already