what are some good places where u can buy spells and staffs and potions

it can be anywhere

and not just stores

Obviously Mages Guild Halls. Each one sells spells, and the Arcane university.

For potions the same. Each city has an Alchemist in a Mages Guild Hall, except for Skingrad which has an All Things Alchemical store across the street and Sinderion in the basemant of the West Weld inn.

Bravil also has a Warlocks Luck shop, and the Imperial City has the Mystic Emporium.

You can also try Edgars Discount Spells in the Imperial City, though some people thing there's something "wrong" about buying discount spellls.

For staffs (staves?) best to find them. Potions too. On Necromancers or Conjurers. Nice staffs can be found on Liches in Undead dungeons.

I never buy potions. You can make them yourself for free or get good ones from necromancer caves.

In addition to the above, I think all the cathedrals in all the cities have priests or others who sell spells.

mages guild for the good spells

and for the very cheap and good for early mages = Edgar's Discount Spells

also

is there any place to buy magic spells in Shivering Isles?

Try Earil's Mysteries over in Crucible (New Sheoth), one house down from Cutter's smithy. He's got quite a few nice destruction, restoration and conjuration spells.
Mirili Ulven over in Highcross only sells a few command creature spells.