Yes, you can create magical corpses
Here are the spells that can allow you to conjure dead bodies out of nothing.
It goes without saying, but for completeness, I'm including it here. This can create a horde of magically summoned corpses that were never alive in the first place, at a 33.33% risk of not being able to cast this spell again.
You can take a humanoid-sized object and transform it into a corpse. If you concentrate on the spell for the whole hour, the transformation is permanent.
By cloning someone, you are creating an empty vessel for their soul to return to should their primary body die. However, if the clone's vessel is ever disturbed, the clone's body stops being inert and starts decaying. This exact duplicate of a (potentially) humanoid creature is, of course, now a corpse a this point.
Alternatively, the primary creature could kill themselves so that their soul transfers to their clone. There may be some incentive to do this because the clone can be a younger version of the cloned creature. Their primary body can then be raised as a zombie.
You can create nonliving vegetable matter, which will last for 24 hours. If you include minerals, it lasts for 12 hours. Depending on where you stand on the "dead creatures are objects/creatures" debate, you can arguably conjure a corpse from this spell. The corpse can then be animated, as long as it isn't a material component in your animation spell.
Similar to creation, depending on where you stand on the aforementioned debate, you can possibly fabricate a skeleton/corpse from raw materials, which you would be able to animate. You can't use this spell to create things that normally require a high degree of craftsmanship, though, unless you can craft that object in the first place -- so you might need to learn how to put together a skeleton/corpse from inert materials with your own hands first.
The Major Image family of spells, plus Illusory Reality
Minor Illusion, Silent Image, Major Image, Programmed Illusion, etc., can all create images of corpses. Combining it with Illusory Reality, these corpses can become real for 1 minute. You can then animate these corpses, and even possibly make their reality permanent. Of course, as you are a necromancer, you will need the help of a high level Illusionist to pull this off.
Do corpses have to be once living?
Marq's answer claims that a corpse needs to belong to someone who once lived, and a clarification about this has also been raised in the comments. However, a corpse is just "a dead body," not "a body that once lived."
A dead body is simply a body without life. "Corpse" is not a defined game term, and a plain English reading can call a body bereft of life, produced scientifically in the lab, a corpse as well.
While it's valid to say corpses are bodies which belonged to once-living people, there are other perspectives. One is that a corpse can simply be a body that isn't alive.