I found a couple places in swamps with drowners near swamp gas.
This is in Velen, south east of the map. You go here for a main quest. Just go around in the red circles and you'll find drowners.
Small circle = 4 drowners
Big circle = 3+ (some spawn near swamp gas, some don't)
This is in White Orchard, North of the map. You go here for a main quest as well.
You can find 6+ here near swamp gas, just roam around.
Those were the only two places I went, but I'm sure there are better spots out there.
So go there, lure some drowners near the swamp gas, and then Igni (or fire bomb, but Igni is easier). Simple as that. After you kill them, leave the area and come back.