Although linoleic acid itself is an important nutritional source of essential fatty acids, there is no evidence that you need to get conjugated linoleic acid in your diet. CLA does occur in food, but it would be very difficult to get the recommended dose that way.
Supplements are the only practical source.
The typical dosage of CLA ranges from 3 to 5 g daily. As with all supplements taken at this high a dosage, it is important to purchase a reputable brand, as even very small amounts of a toxic contaminant could quickly mount up. There is some evidence that CLA might help you lose fat while retaining muscle. However, what we know is based primarily on some interesting animal studies and very small human clinical trials. 1,2 Better studies in humans are currently under way, but results are not currently available.