In book six, Harry Potter and the Half-blood Prince, the Horcrux is surrounded by a lake filled with Inferi. Did Voldemort have to use people he killed, or did Voldemort simply just go to a graveyard to procure his army of Inferni? As an example, we read in book seven that Dumbledore says that for Grindelweld the Resurrection Stone meant an army of Inferni. Does that mean he was going to have to kill a lot of people to create an army, or could he just go grave robbing?
The Inferi at the lake were homeless Muggles murdered by You-Know-Who
As described here:
The Inferi whom Harry and Dumbledore encounter in the depths of the lake in Harry Potter and the Half-Blood Prince were, when alive, mostly vagrant, homeless Muggles whom Voldemort had murdered for the purpose during his first rise to power, although some were the earthly remains of wizards or witches who ‘disappeared’ without explanation.
The second part of the sentence above: some were the earthly remains of wizards or witches who ‘disappeared’ without explanation indicates a much darker side. You-Know-Who actually dug up the remains of dead wizards (or witches) as well to create Inferi.
Seeing what You-Know-Who used for his Inferi army to guard the Horcrux, it is clear that any body can be turned into Inferi (Muggle, wizard, recently deceased or long dead people as well).
Grindelwald actually had plans of using the Inferi as an army
As mentioned in Book 7, Harry Potter and the Deathly Hallows, Dumbledore tells Harry that Gellert Gridelwald and himself long ago tried to collect the three Hallows. For them,
The Elder Wand meant limitless invincible power.
The Resurrection Stone was the one they both wanted because for Dumbledore, it meant he could finally see his sister and apologize for what happened. And for Grindewald, the stone meant that he could raise an army of Inferi.
However they never really thought of the Invisibility Cloak and they only considered getting it so that they could complete the Hallows.