Never quite made it to 300 cooking in vanilla.
Have done so in Classic.
My cooking still seems really incomplete in terms of recipes or high level food, despite getting 300. I have visited all the main trainers.
Are most of the recipes from rare trainers or quest drops?
mostly vendor and quests.
There really aren’t many great recipes in Classic WoW. Here’s a good guide:
https://classic.wowhead.com/guides/wow-classic-best-food
Cooking was neglected in vanilla, it is major buffed in TBC.
Stuff level 60s buy
Nightfin soup
wintersquid
deviate delight
Whatever the sandworm meat stuff is.
Personally I use mightfish for tanking and poached sunscale salmon for levelers and warlock.
Everything else is a waste of time.
The caravan in Desolace has several recipes, with some unique ones depending on which side of the patrol they’re at.
There are like 3-4 total.
One comes from blue scepter quest line, or can be bought for a lot if the player sells it.
Sandworm meat, trainer in silithus.
Night fun soup
Wintersquid.
Deviate delight - fun recipe.
That’s it. Rest are just for leveling and feeding your cat pet as hunter.
All of these replies are very helpful, thank you guys.
I really don’t mind if it’s only a few recipes. Since you can only have one well-fed buff, that’s ok with me.
I appreciate all the help. I will find out how to get all the recipes you all mentioned.
Don’t forget tuber surprise! 
Stam food is cheap enough that it is a good bg staging buff.
This^^ pretty much sums it up. There are a gazillion recipes, and almost all of them are worthless except for leveling, like any other profession, I suppose. But some completionists like to try to collect them all.
Completionist Pro Tip: On AH, click the filter on the left for the profession you want to search, e.g. Cooking. Then at the top there is a little check box for “Usable Items.” If you check that box when you search for recipes, it will ONLY show the ones that you haven’t yet learned.