According to VeganHealth.org, "Without diet planning, vegans and vegetarians have low omega-3 intakes and blood levels. Therefore, vegetarians and vegans should moderately supplement with sources of ALA and also supplement with DHA."
Essentially this translates to their three basic recommendations:
1. supplement your DHA by 200 - 300 mg
2. Do not prepare food with oils high in omega-6 (corn, soy, safflower, sunflower, "vegetable," sesame oil). Instead, use low omega-6 oils like olive, avocado, peanut, or canola. Only cook canola under low heat and for short periods.
3. Add 0.5 g of uncooked ALA to your diet daily. This would be the equivalent of:
1/5 oz walnuts (3 halves)
1/4 tsp of flaxseed oil 1 tsp of canola oil 1 tsp ground flaxseeds
Regarding the first recommendation, I've been using the Spectrum Vegetarian DHA. There are several other brands that I am sure are comparable, but you have to buy them online and some of them are a little more expensive. The Spectrum brand can be found at Whole Foods. I haven't seen it anywhere else yet. Some people think the only way to get the DHA is by eating fish or taking fish oil. Not so.
The theory with algae-sourced DHA is that the fish get their omegas from the algae they eat, so why not go straight to the source? That way you leave the fishies alone and avoid any potential heavy metal toxins. And I have to say that I'm pretty sure the Spectrum Vegetarian DHA is at least partially responsible (in addition to a little bit of added exercise) for lowering my high cholesterol by like 50 points in just 3 months putting me at a decent number. I mean, I didn't follow the scientific method or anything, but that was like the only real change in my diet.
I think the second recommendation is pretty self-explanatory. Most vegan recipes I encounter use the canola and olive oil, and to a lesser degree the peanut. Though, I do also see the sesame oil (and toasted sesame oil), and I actually like to use that sometimes too.
For the third recommendation, it's really easy to get those ALA omega 3s. I add walnuts to salads or eat them plain as a snack. If you cook with the canola, well, there you go. I like flaxseed meal for some baking. It's an excellent egg replacer, especially for breads. I also eat Nature's Path Flax Plus Oatmeal that has added flaxseed which you can find at Sprouts, Fry's Marketplace, and Whole Foods.
You can cook with the seeds and meal, but do not heat the flaxseed oil (it goes rancid easily). I personally think the flaxseed oil alone tastes kinda nasty, but I know some people like it on salads or in mashed potatoes. Arbonne's NutriMinC RE9 REsist Essential Fatty Acid Dietary Supplement is the only product that I am aware of that has flaxseed in vegan capsules where you don't taste any oil, but it's got a few other ingredients you may not want (CoQ10, etc), including a hefty price.
Also, chia seeds are becoming quite popular and have lots of ALA. I have not tried them myself, but they sound intriguing. Here's what Dr. Weil has to say about chia seeds.
Ok, so don't forget, I am not a doctor or anything. Please refer to your own doc or nutritionist for your specific needs.