Burden can be obtained at lower levels, and always available at Edgar's Discount Spells. Members of the Cheydinhal Mages Guild sell Alteration spells, including leveled Burden spells. Spell Merchants at the Arcane University may have access to this spell effect. Once purchased, custom spells containing the Burden effect can be created at an Altar of Spellmaking.
The Cheydinhal Recommendation offers a tutorial of Feather, waterbreathing, and burden. Vidkun, the guildhall's previous associate, drowned because he wore a Ring of Burden underwater. First appearing in The Elder Scrolls III: Morrowind, the spell was eradicated from The Elder Scrolls V: Skyrim, along with feather and open.
To maximize effectiveness, it should be paired with Drain Strength, which lowers the weight limit on characters. Though not the most useful spell, many enemies utilize it to help bring you down. It's true usefulness shines when used in poisons, along with a Burden and Drain Strength enchantment on whatever weapon you are using.
It is possible create a custom spell at an Altar of Spellmaking, which casts Burden on self. This can be used underwater to kill oneself.