This recipe is now trainable, rather than being a vendor recipe.  As stated above, apparently the Alliance trainer is in Feralas, and the Horde trainer for it is Rogvar in Stonard.
I searched Darnasus for this and couldn't find it. Finally after searching other WOW sites I found a MASTER ALCHEMIST that trains this potion in Feralas.
I am baffled by the misinformation on this subject. Could Blizzard have changed something? This potion is NOT available through Algernon in UC. However it can be purchased from Rogvar the Master Alchemist Trainer at Stonard in the Swamp of Sorrows. He is up the stairs in the building next to the wind rider master. Do not be confused and try to perchase it from the alchemy supplies vendor. If you went there you're in the wrong building and he doesn't have it. You must go to the trainer to get it, NOT the vendor. I looked for this from Algernon in UC for a month all he ever had was shadow elixer recipe. Don't waste your valuble time. Go see Rogvar in swamp of sorrows.
I've been checking Algernon every day for two weeks now.  All he has is a variety of vials. Why does Blizzard change this stuff around?  LOL this is maddening.  If you have any good clues for a mid-40's character with alchemy, please be kind enough to post it.  I'd be very grateful. 
This reciepe is found at the alchemy supplier Ulthir in Darnassus at Craftmen's Terras. When I first got there he hadn't got any in stuck, but then i just walked away for 2 minutes and then he had 2 of them :)
I've been periodically checking the vendor every 30 minutes or so for a few days cuz I'd rather not pay 12g to the people sellign this at AH on the server I play on, but it doesn't seem to be restocking it, doesn't seem to be restocking free action potions either for that matter (though I already know that one so it doesn't matter to me) Anyone have any idea whats goin on?
I've now bought two Elixers of Shadow Power and haven't seen either of the other two recipes and I haven't seen anyone else approach Algernon (UC) to buy them. /
I was on really early, so I know no one was there, but Algernon didn't have the mana spell to sell.  So I took the advice of the previous post and walked away. 
Not sure if it's been mentioned, as none of my previous comments have worked, but the Superior Elixir of Mana Recipe can be found at Darnassus, from the Alchemy supplier named Ulthik (or something odd like that).
Seems the Horde vendor selling this doesn't respawn the recipe if the other recipe he's selling is available. Either that or he's not regenerating the recipe at all. Normal respawn should be 30 minutes but he never has this in stock.
Checked him out numerous times, even went has far as staying there 30+ minutes and nothing :(
And to think i sold it on AH when i got one back 3 weeks ago :(((
I had to buy two shadowpower recipes over a span of twenty minutes , then come back in ten minutes and algernon in UC had the recipe for superior mana potion.
I'm a 33 Ferral Night Elf Druid whose Alchemy skills are topped out at 225/225. 
I've visited Ulthir in the Craftsmen's Terrace, Darnassus about 15 times over the last week.  I can always find 1 or 2 Recipes for Great Rage Potion and three times I found the Recipe for Free Action Potion, but I have not been able to find the Superior Mana Potion Recipe. 
Can anyone truely verify by going to Faralas that Blizzard has changed this to a trainable skill by a trainer in Feathermoon, rather than a consumable that can be purchased from a vendor in Darnassus? 
Thotbott notes that Ulthir and Sylvanna Forestmoon in the Craftsmen's Terrace both sell it for Alliance members, but Sylvanna is a trainer, not a vendor.  Other posters have stated there is a trainer in Faralas for this skill, but Faralas is a 40-50 level territory.  How do they expect a level 33 to get there except as part of a group and this "low" level skill should not be considered a quest reward for forming a group to run through Faralis just to take a boat to Feathermoom for alchemy/mana potion training.
Limited Supply in Darnassis from Ulthik, or whatever his name is.  Was initially camping there for a rage potion, but hey, found this instead.  Uploading thott data later =)
Okay.. I have all of the ingredients for this potion, but... gulp... the recipe is only in Darnassus?  Or is it really true that it's available in Undercity? Horde certainly have a resource. Right?
I've looked all over both here and on Anonymous, and have found loads of information on the recipe for the Superior Mana Potion, but have found absolutely nothing on the recipe for the Superior Healing Potion. Does this recipe exist? If so, where do I get it?