Looking for a good list of vendors who sell these and not just the standard/beginner ones either.

All the lists I find belong to some sort of wiki and they tend to mix Morrowind/Oblivion/Skyrim locations and characters.

Also, the higher expert ones, can they only be purchased by joining the Mages Guild and getting the members to sell them? (The "above 90" ones.)

The level of spells for sale will increase as you level that school of magic.
The palaces of the Jarls will have court wizards that will sell a variety of spells. The top level spells will only be available from the college after you complete each of their little quests.

You have to join the College of Winterhold to get the master level spells.

Thank you! I'm having a hard time leveling alteration and thought more powerful spells would increase that skill faster. Bummer I have to train it first, then get the better spells.

About master level spells: that seriously svcks. This character is the Dark Brotherhood Listener and quite the illusionist, why would he have to join those losers' college in order to get those spells? Grrrr.

You can loot the Transmute spell in Halted Stream Camp. Turn iron ore into silver and gold. Level alteration and profit.
Detect Life can also be obtained as a reward. Treva's Watch in The Rift. There's some guys camping outside. Help them take back the fort and you the get the spell book.

The master level illusions spells aren't that great. I just usually dual cast the expert spells. With the perks they are effective. I used dual pacify on a dragon priest earlier.

Everything spell-related can be bought in the College of Winterhold from the 5 teachers and what books they sell depends on your skill level (40 for adept books, 65 for expert)
When you reach 90 you can ask them for an extra quest which will allow you to get the master spells (and, as viinse said, are completely useless)