In general you can get Omega 3's from fish, so a dry or wet diet that includes fish should work. Some foods include fish along with other protein sources, others are allergy formulas and have fish as the only protein source with something like potato or rice.
In the past I gave my dog 3V's Derm-caps, an omega-3 supplement, because the vet said he had dry skin and needed more than he was getting from food. I wasn't happy with the results. He just gets Omega 3's from his food.
I agree with nodogyet, get the supplements first. It would be easier than trying to switch a diet. Also i don't believe that you can give too much omega 3. I would do a teaspoon a day for a smaller dog and tablespoon for a large dog. You can always add more if you need too.
Thank you all for the input. I found a good food by Solid Gold, "Just a Wee Bit" and when I e-mailed them about the omega-3 content, I was advised that all their foods contain a minimum of 1.5% omega-3. They also have a canned food called "Turkey & Ocean Fish" which I could mix with the dry since she is a finicky eater. Have any of you had any experience with this food?