While the Arts of Necromancy are only illegal in the province of Morrowind, few citizens of the Empire have an enlightened view of our Art. Thus, the acquisition of corpses on which to experiment is often difficult.
In Cyrodiil, a few Necromancers who have served the Empire are given the corpses of criminals and traitors to use legally. This provides those who have acquired such a post with a fresh supply of corpses, most of them young, strong, and intact.
In Morrowind, the outlawing of Necromancy would make its practice impossible were it not for the fortunate institution of slavery. While the Temple will investigate obvious signs of Necromancy such as hastily emptied graves or ash stolen from one of their ashpits, a careful and discrete Necromancer can thrive in Morrowind by taking slaves at a modest rate. Most will assume the slave escaped or died in the Ashlands.
Finding suitable corpses in Black Marsh is nearly impossible due to their rapid decay. There are also diseases, Argonian tribesmen, and other difficulties that must be dealt with. I know of only a few Sload Necromancers who operate successfully in Black Marsh, and even they stay near coast.
While the forests of Elsweyr pose some of the same problems as those of Black Marsh, the deserts preserve corpses for hundreds of years in a way that requires very little preparation. Khajiit of the desert tribes are often buried with only a small cairn of stones which are easy to find and uncover. The Khajiit show remarkably enlightened indifference to graves being uncovered. It is said that in the port of Senchal, one may purchase anything one desires. This is true if you desire fresh corpses.
While few Bosmer perform Arkay's rituals when burying the dead, the more primitive Bosmer still practice cannibalism upon their enemies, which reduces the number of available corpses. As would be expected from such a backwards people, they have an intolerance of Necromancy that goes beyond all reason. Many Necromancers who practice our Arts in Valenwood become "one with the trees" themselves.
Summerset Isle is even worse in some ways. Some Altmer born into the most respected noble and scholarly families are actually allowed to study the dead in the open. Their research, however, seems to be centered on finding ways to extend their lives even further rather than the more practical uses of our Art. A Necromancer of any other race caught in Summerset Isle can expect the worst possible punishments.
In Hammerfell, where worship of Arkay is strongest, the dead are almost always subject to Arkay's Law. There are exceptions after large battles or in remote areas where death occurs far from meddlesome priests. Fortunately, the dangerous terrain and creatures in the deserts and mountains of Hammerfell makes the acquisition of corpses possible, though they are often in poor condition and require special care in preparation.
The newly formed Orsinium presents a unique opportunity. As you know, Orc corpses are among the most sought after for the durability of their skin and the strength of their bones. If King Gortwog will listen to reason, we could offer the services of our Art in defense of his young nation in exchange for disposing of the Orcish dead. A mutually beneficial arrangement as I'm sure the Orcs will agree. To this end, a delegation has been sent to Orsinium, though we have not yet heard any word on the state of these negotiations.
In my native High Rock, traditions dating back to the witch kings and nomadic horsemen mandate cremation of the dead. This is practiced almost without exception in the north, through an Imperial burial in a tomb or city cemetery is more common in the south. There are still many corpses easily taken from the battlefields of the War of Betony and the lawless times that followed. There are even rumors that King Gothryd of Daggerfall may institute the Imperial practice of donating the corpses of criminals for Necromantic study as a deterrent to the bandits and pirates that still threaten the Iliac Bay.
In Skyrim, the cold weather and isolated terrain allow a few Necromancers to operate freely. Alas, the availability of corpses is limited to Nords who die from exposure or in battle. While the cold is preservative, the snow makes these corpses difficult to find. More research dedicated to the magical detection of corpses would be invaluable to the Necromancers of Skyrim.
The Sload are the most famous Necromancers, but little is known of their native Thras. In Tamriel, Sload only practice Necromancy on other races. It is uncertain whether this is true in Thras as well. If so, it would explain the number of slaves that are purchased in Tear by Sload merchants and the rumors of Sload airships carrying corpses from Senchal.
These difficulties lead many Necromancers to create their own corpses. While I prefer to work with those who have died a natural death, a more expedient approach is sometimes necessary to further the study of the Art.
While the Arts of Necromancy can be practiced on animals, such experiments rarely produce interesting results. The servant's ability to follow directions seems to be related to the subject's intelligence in life. While raising the corpse of a man, elf, or beastman can produce a useful servant, the corpses of animals produce mere guard dogs at best. Often a raised animal is unable to distinguish its master from the rest of the living and many amateur practitioners have been torn apart by the animal servants they created. Let such stories be a lesson to you.