I had asked a similar question earlier and was told that most buy kents garlic at the lfs. But you can ask NM reef how he does his because I read in one of his posts that he uses fresh garlic from the grocery store. HTH
Garlic extract from a health food store works great. A little pricey, as is Kent's, but it goes a long way. Just make sure you don't get garlic oil. GNC I know carries it.