The Witcher
Harvall is a steel sword which can be found:
- In Chapter II, or III, in the swamp on a decomposing body on Wyvern Island.
- Or in Chapter IV on a corpse near the wyverns in the Fields, north of the Small farm (though it seems it is there only the first time you visit the area). It doesn't seem to appear if you already looted it from the corpse in the swamps.
- It can also be obtained by completing Sabrina Glevissig's mission in The Price of Neutrality premium module.
It is a much better blade than the default witcher's steel blade, and probably better than any meteorite sword you may have at that point in the game. (You should compare the stats just to be sure.) Regardless, if you find the sword in Chapter II, definitely compare it to your current steel blade and sell whichever one has the worst stats, keeping the better one for yourself.
The Witcher 2: Assassins of Kings
Harvall can be bought from Polycarp of Rinde, outside Loc Muinne. It is unclear if an imported save game with the Order path is required, however upon approaching another knight of the Order in the Keadweni camp, he makes reference to Geralt's part in the death of Sigfried. Also with no import the Knights of the Order outside Loc Muinne are hostile to Geralt on sight, and will not allow him into their camp. It is likely that Geralt would have had to side with the Order in the first game and import.
The Witcher 3: Wild Hunt
Harvall returns in the third game and can only be found in four quest-related locations.
The first and easiest to access is during the treasure hunt quest An Unfortunate Turn of Events, and the second is another treasure hunt, A Plea Ignored, which however involves a much higher level enemy.
The other two chests unfortunately require Geralt to have reached level 30 before Harvall will appear in them (but it is guaranteed at that point), even though the locations are usually visited much earlier, so you may want to leave them to loot later:
- Up in the lighthouse at Lornruk, next to Dobromir's journal from Scavenger Hunt: Griffin School Gear.
- Behind a breakable wall in the sewers underneath The Kingfisher Inn. You will gain access to this area during Now or Never, and the spot in question is just before reaching the underground library.