I have a mage in my party that has the ability to cast 5th level mage spells from his spell book. I just finished Dragon's Eye and when I talk to the guy that sells scrolls in Kuldhar he doesn't have any 5th level spells. Can anyone tell me where I find Cloud Kill, Monster Summoning 3, Hold Monster or any other fifth level spells?

There are some fifth level scrolls near the end of the area you're about to start, Severed Hand. You find some in one of the towers there, for instance. And there are more at the beginning of the next area, Dorn's Deep. There's a fellow there who sells a few fifth level scrolls and some potions.

As for the specific spells on your list, I have a party partway into Dorn's Deep right now (my third) and the only one of those they've seen is Cloudkill. They have all the Conjure Elemental spells and Summon Shadow, though. Also Cone of Cold. Feeblemind was for sale but I didn't buy it. Don't think I've seen anything else fifth level yet.