I realize I can just make these myself, that I can visit a temple for cures, but I haven't found anyone who sells these three spells. Have I missed someone? I've tried to visit every spell merchant in the game.

Also, why did they limit player-created spell magnitude to 100 in this game, while there are still spells well over that limit?

You can buy all of the restore spells in the chapels. Each chapel has a person who will sell one restore spell (like Willpower in Skingrad). Trevaia in Anvil is the only one who sell two spells (Luck and Personality).

Sometimes they only sell at particular times and are difficult to buy from. The two most difficult are Ohtesse (in the morning she's out around the river by the mages guild hall, and at noon she walks to the chapel, You need to catch her while she is walking to the chapel), and Trevaia, more details here:

http://www.uesp.net/wiki/Oblivion:Trevaia

http://www.uesp.net/wiki/Oblivion:Spells

Trevaia is dead, killed by the Knights of the Nine add-on.

If you have one restore attribute spell and access to a spellmaking alter, you can make the rest of the restore attribute spells yourself.