How can you buy a health potion in oblivion

You can buy some in different shops that sell potion ingrediants. But its far easier and cheaper and better if you make you own.

Just take a mortar-pestle and stick in two foods like apples or something and combine them to make a health potion. They have to have two of the same 'qualities' to combine.

If you have all 4 potion making things and use better foods, your potions become much better.

As you get better in Alchemy, it shows you more ways that food can be combined.

You can make a lot of gold this way also. Just pick food from peoples gardens and combine everything and then sell what you dont want back to the store.

--edit--
Also, Alchemy is one of the easiest things to upgrade since you find food things and alchemy ingredients all over the place, especially if you are doing thieves guild quests and are breaking into a lot of homes and places.
Its much easier than upgrading something like heavy armor or light armor, which take monsters beating on you to upgrade.
and easier than lockpicking that takes lots of chests to open but they are not as common as food and flowers and bushes and stuff.