Items needed

Ethers, max ethers, elixirs and max elixirs can be found around sinnoh, but there aren't many.

Get a Pokemon in your party that has the ability 'pickup' because it will pick up items randomly and after battles... And the higher the level your Pokemon, the better the items it picks up.

You can also grow leppa berries. They will restore 10PP to one move of a pokemon.

The three Pokemon you mean are able to be found after completing a part of the storyline. They are called Mesprit, Uxie and Azelf.

They will be at the three lakes. Two you will be able to battle at their lakes... Azelf (the blue one) is at level 40. Uxie (the yellow one) is at level 40. But when you go to the lake nearest your home town, you will try to battle Mesprit (the pink one) and it will run away. It will then be roaming around the grassland of sinnoh, so when you encounter it, it will likely run away again. If you want this Pokemon, use your master ball on it.

I recommend that you save before each battle, and take lots of ultra balls and dusk balls. (i mean lots... 30 of each or more if you can)

There is one other roaming Pokemon that can be caught in this game, it is Cresselia, so if you really want that one, too... Think first before using your masterball on Mesprit.

Mesprit uxie and azelf are all lvl. 50, not 40.

Items like the Ether, max ether, elixir and max elixirs, are rare, and there are only a few of them in the entire game, so use them sparingly when you find some. So, because of the fact that the items are so rare, they will all be hard to find.

You can look underground, and you might find some extra Elixirs, or such lying around underground, but I don't know for sure if they can be found under there. Capture, and train a Pokemon that knows pick-up, this is lucky itself, because with pick-up as an ability, the Pokemon will find special items after each battle, so you can use that to look for Elixirs, or ethers, or max ethers, or whatever else you want. Leppa berries also restore your pp(power points) so look for those, and grow some if you have the time to do so. As for the three Pokemon you seek. They are the three spirits of human emotions, Uxie, Mesprit, and the last one is Azelf. The three are legendary, and are fairly easy to catch. All three of them are on level 50, and are psychic, after beating Team Galactic, go to each one of the 3 great lakes, and you will encounter a spirit that you can kill, or catch. One of the spirits, I think Uxie, is a pink spirit that will run aweay when it sees you so you will have to run around Sinnoh looking for it if you want to catch it. The three Pokemon are easy to catch, but you might want to bring around 80 or so Ultraballs just in case things get tough. Hope this helps.

I think you can get ethers at the big Pokemon store

Just a note from the previous answers: Ethers and Elixers CANNOT be bought at stores, or be found Underground. They can, however, be picked up by Pokemon with the "Pickup" ability. But this is rare.

The three Pokemon you're refering to are Uxie, Mesprit, and Azelf. They can all be caught at level 50. After you fought Dialga on the top of Mt. Coronet, those Pokemon can be found in the caves at their corresponding caves. You'll just battle Azelf and Uxie upon interaction, and using Dusk Balls is best to catch them, but when you talk to Mesprit, she'll run away. She's found in random routes in [continental] Sinnoh. You can track her down with the Marking Map app on your Poketch. However, when you do run into her, you make a move and she'll run away the first chance she gets. You need a Pokemon with Mean Look to keep her in battle. Then, weaken her and use Dusk Balls (if it's night), or otherwise Ultra Balls.

Look, I don't know about the Elixers, but I know that to catch the lake Pokemon (Mesprit, Azelf and Uxie) after you beat Cyrus and free the trio, then beat all the Team Galactic guys, you can use Surf and go to the middle of each lake there will be a cave go in and save and battle the Pokemon but Mesprit runs away you have to track it. The three Pokemon are all level 50 when you battle them. Hope this helped.