a good brand is Nordic Naturals and you can buy it at health food stores or speciality vitamin stores.It';s really good. They have a website and they have done a lot of research on the products. very pure.
Stay away from teh cheaper brands, they are often rancid and cause more harm than good.
Costco carries a "Pure Alaska Omega" from Wild Alaskan Salmon Oil. From what I have read, thats the best way to go. I personally use it. But - its a big pill. If you can find something smaller go for it - but try and find one from wild salmon and avoid stuff from the small fishes. By the way, don't expect it to do much to help the ADD, but it certainly is a healthy way to go.