In The Elder Scrolls IV: Oblivion, Fortify Skill spells are not available as default spells you can simply buy from regular vendors. Instead, they are usually acquired through special means or DLC content:

• You can buy some Fortify spells from Restoration-specialist mages in the Anvil Mages Guild or from healers in various churches, but these typically do not include Fortify Skill spells specifically

• There are no built-in Fortify Skill spells sold by normal merchants. To get access to Fortify Skill effects for spellmaking or enchanting, you need to acquire special powers or spells from quests, stones, or DLC content

• For example, the Apprentice Stone near Skingrad grants the "Void Seed" power, which allows you to create Fortify Skill spells via spellmaking

• The Skeleton Key (from Nocturnal’s Daedric quest) also grants access to Fortify Skill effects as long as it is in your inventory

• The official plug-in "Thieves Den" adds vendors who sell spells with Fortify Skill effects

• You can also upgrade the Thieves Den hideout to unlock several vendors selling Fortify Skill spells

• Some quests and powers (like the Origin of the Gray Prince quest or Dark Brotherhood's Night Mother's Blessing) grant Fortify Skill abilities

Summary of where to get Fortify Skill spells:

• Acquire the Apprentice Stone power (southwest of Skingrad) to unlock Fortify Skill for spellmaking

• Complete the Nocturnal Daedric quest to get the Skeleton Key, which grants Fortify Skill effects

• Visit the vendors in the Thieves Den plug-in after upgrading it

• Check special quest rewards like Origin of the Gray Prince or Dark Brotherhood quests

You cannot simply buy Fortify Skill spells from normal merchants; you must obtain access through these special means to then create or buy such spells. This approach allows you to craft custom Fortify Skill spells or enchantments to boost skills like Acrobatics, Blade, or Mercantile temporarily