I wouldn't quite look at it in such a black and white way; DHA and EPA are just 2 of the 6 omega-3 fatty acids; fish oil just happens to be a great source, and plant sources don't contain these.
You still want to get other sources of omega-3 into your diet, such as flax, walnuts, and leafy greens (although, for the life of me, I can't really recall what "leafy green" contains any omega-3's)
As far as supplementation, goes, though, I think fish oil and a clean diet would cover your bases. If you were at all concerned, Udo's would be the best choice for plant based EFA's