You CANNOT purchase these from the Midsummer vendor until after you've completed Torch Catching at which point you will receive 5 Juggling Torch as a quest reward and then be able to purchase more at the aforementioned vendor.
You CAN purchase these at the auction house (you shouldn't have to pay more than 5g a piece).
You CANNOT attempt this achievement unless you have completed Torch Catching - your torches will simply explode on the ground as if you weren't actually catching them. If you want to save your Burning Blossom, do the quest, then purchase the balance of torches you need through the AH.
You will need 10-20 torches in your inventory depending on your lag in Dalaran (I failed miserably with 10, but 15 did the trick for me). You need to juggle 40 torches for the achievement, but remember you catch the ones you throw up once they come back down, so you don't need to have 40 in your inventory - and you need to stand in one spot for this obviously.
You will best achieve this by following all other post suggestions here and tossing the torches to yourself by targetting your feet - if you don't 'catch' your torches, they will be destroyed by exploding on the ground. Add-ons are helpful, but not necessary.
The second you score your achievement , DON'T MOVE... Catch the rest of your torches and hand 'em to a guildie or throw them on the AH. You will never be rich (in wow gold) if you are wasteful.