The foods – flax, chia, walnuts, soy – provide ALA omega-3; ALA does not convert well to EPA and DHA, which are the more functional omega-3s (converts best to EPA).
Conversion is the issue, or as Dr. Joe Hibbeln would say, how much is in “the tissue IS the issue”.
A genetically modified (GMO) omega-3 in soybean oil, offering SDA, is coming to market shortly, in foods. SDA converts better to EPA, but still not to DHA. The GMO n-3 will be marketed as “natural, sustainable, land-sourced”.
One needs to eat a good amount of flax to get a good amount of ALA. It is not hard if you find a way to get it regularly into your diet. The least expensive way is to buy the seeds, keep them in the fridge and grind as needed (a dedicated coffee grinder is a good idea). You have to chew the seed to get the omega-3; you can also use flax seed oil. Some ideas: use flax seed oil-based salad dressings; add freshly ground flax to blender drinks, add to quick breads or cookies, or mix into peanut butter for toast (yummy); snack on walnuts and add them to salads.
There is no veggie source of DHA (DHA is most important for eyes and brain), but you can buy the algal form in a capsule, and some companies are blending ALA with veggie/algal DHA.
Let’s stop this rumor: Seaweed used in sushi does not contain DHA. It’s a myth.
There is new thought that vegetarians may convert to EPA and DHA better than meat eaters, but we don’t know for sure.
Being vegetarian, and eating occasional fish is a good solution.
Tilapia (type of fish) is popular and inexpensive and a very low source of n-3 (it’s more like chicken than fish).
There is no RDA for omega-3s (there is an Adequate Intake for ALA of 1.6 grams). When we have an RDA it will probably be 250 mg EPA and DHA.