Hi, the best vegetarian/vegan source of DHA is straight from the source! This means getting it from where the fish get it

.
Algae.
Go to www.froogle.com and search for "VEGAN DHA" - it is sold in tablet form and you can get 200mg from each tablet.
I can understand why you want to get DHA, especially since our brains are composed of 30% DHA and research has found that many untreatable forms of depression, aggression, PMS symptoms are greatly alleviated by supplemental DHA.
I am waiting for my own DHA supply to arrive this week.
Flaxseeds convert Omega 3 & 6 into DHA at a rate of less than 5% --and even this number depends on stress levels, genetics and our bodies. This may not be an efficient enough process for you if you have any of the above symptoms or want to supplement your diet if pregnant. Fish oil contains 1200mg of DHA, so it will be difficult to overdose on the algae tablets

. The American diet has been found to be almost completely deficient of DHA and symptoms like depression, panic attacks and other disorders linked to this nutrient are skyrocketing. Too little DHA causes the cells in the brain to become too rigid (so that "messages" can't pass through) and too much causes the cell walls to become gooey and leaky. So, I'm sure you'll find the proper daily dosage.
It is great to see someone is doing their own research for their own health!! GOOD LUCK!!!