You can buy garlic extract at pet stores (I know for sure that my local Petco and Petsmart sell it), GNC/health food stores, or you can make your own (just press garlic, and save the oil. Dilute the oil with some RO water and you've got homemade garlic extract).
Garlic (just like in humans) boosts the immune system and has antimicrobial properties. In fish, it also stimulates appetite, so that's an added bonus if you have a sick fish.
You can buy vitamins at any pet store as well. Kent Marine makes a good one (I can't remember what it's called exactly), and another popular brand is Selcon, which you have to buy at an LFS or order online.
Prevention is better than treatment, IMO. If you keep your fish garlicked and vitamined up, stress free, and your water quality high, your fish won't ever have ich outbreaks.
You can soak any type of food in the vitamins and garlic. I thaw out my frozen food in a cup, and squirt the garlic and vitamins on it, and let it soak in for a few minutes before I feed it. You can do the same to pellets or flakes if you feed those.