good question,
I know swanson vitimans has a buy one get one free
on malic acid rite know for $15.
I was thinking on buying for soften gallstones
skyblid
If you give your location/continent that might help with recommendations. I get mine from Tradeingredients (UK) in bulk £9.95/Kg or £24.95/3Kg delivery on top so I try and combine my orders.
I do not know any suppliers in the States, I suggest that you do a search on google for "Malic Acid Powder" and "Bulk Malic Acid Powder" and see what it throws up. Body building suppliers sometimes sell it and there should be quite a few of those in the USA. Try eBay too. I would not waste money on capsules as it makes it far more expensive than it should be.