To buy the Soul Trap spell in The Elder Scrolls IV: Oblivion (including Oblivion Remastered), you need to visit any Mages Guild chapterhouse in Cyrodiil. Most cities have a Mages Guild where you can find NPC vendors selling the spell. For example, in Chorrol, you can buy it from Alberic Little for around 360 gold

. Other Mages Guild vendors who sell Soul Trap include:

• Alves Uvenim in Leyawiin
• Delphine Jend in Bravil
• Druja in Skingrad
• Selena Orania in Bruma
• Sulinus Vassinus in Skingrad
• Thaurron in Anvil
• Trayvond the Redguard in Cheydinhal
• Ungarion at Warlock's Luck in Bravil
• Agata in Leyawiin (sells Greater and Superior Soul Trap versions)
• Aurelinwae at Mystic Emporium, Imperial City (sells Superior Soul Trap)

You do not need to be a member of the Mages Guild to buy the spell, but your Mysticism skill level must be high enough (at least 25 for the basic Soul Trap spell) to cast it effectively

. Soul Trap is a Mysticism spell that traps the soul of a defeated enemy in an empty Soul Gem if the enemy dies while the spell is active. This allows you to recharge magical weapons and craft enchanted items

. In summary:

• Go to any Mages Guild in cities like Chorrol, Leyawiin, Bravil, or Imperial City.
• Find the spell vendor (e.g., Alberic Little in Chorrol).
• Purchase the Soul Trap spell (cost around 360 gold for the basic version).
• Ensure you have empty Soul Gems to trap souls.
• Cast Soul Trap on an enemy and kill them within the spell duration to capture their soul.

This is the most straightforward way to acquire the Soul Trap spell in Oblivion Remastered