I found feeblemind against mages very useful (probably not in the later stages of the game, but in the city). I don't know what level you are, but since you're seeking high level spells I assume you're not a beginner
I see that all of the merchants were listed above, so I'm not gonna repeat that. I just give a few useful tips for battling against mages, if you're interested:
So, Khelben's Wharding Whip will make defeating a mage much easier. It not only removes one powerful defense each round but prevents the mage from casting spells, so probalby one of his spells per round will fail. That's cool. Removes all of his spell protection in no time, so he will be vulnerable - now you can lower his MR, and then use one of my favourite spells, Finger of Death

(No, I'm not a necromancer!

)
Time Stop, of course is the top of all

You gain enough time to remove all spell protections, summon monsters, if you want, and so on. But if you face someone who's immune to it, then you're in trouble...
SPOILER-->
The final battle (ToB) with Melissan went really bad b/c I casted Time Stop. She was immune to it, so I stopped all my party members while she could do as much damage to them as she wanted, and conjured up several pit demons... Uh...
Good luck! lol