Which ones are those? I know people are taking flaxseed and rapeseed oil but that's mostly ALA which is hardly converted to DHA by the body.
Usually EPA and DHA coming from 2 different cultures are mixed into a commercial "algae oil", together with some vegetable oil.
Most of those unicellular organisms are not algae, but there is no better name that could be understood by the general public.
For example, EPA is extracted from thraustochytrids, which are non-photosynthetic unicellular distant relatives of the brown algae.
The only problem with the "algae" oil is that for now it is very expensive. In Europe it is 8 times more expensive than fish oil. While I eat mostly vegan food, I make a couple of exceptions and one of them is fish oil. I have tried algae oil. It was good, but I could not afford to pay for an adequate daily dose of it.
I do not know how much of this premium price is due to the cultivation costs and how much is due to vegans accepting to pay more for a vegan solution.
https://www.amazon.co.uk/dp/B07BRY1FRZ
I also saw normal fish oil in similar price range but that could just be Amazon inflating prices?
At your link, the price is GBP 17 per 24 g DHA, which results in about EUR 0.82 per gram of omega-3.
I have paid about the same price, i.e. 8 times higher than fish oil, for a similar algae oil.
So the price seems right. However the product from your link has a problem, it contains only DHA.
There are 2 omega-3 acids that are required by the human body, DHA and EPA. Fish oil contains both and also vitamin D.
The cultivated algae produce either DHA or EPA, not both. Some vendors mix oils from 2 algae cultures, to sell supplements with both DHA and EPA.
Others sell DHA and EPA separately, like in your link.
If you choose this DHA supplement, you must also search an EPA supplement, to take them both.
Otherwise you must search a supplement with both DHA and EPA. The prices are about the same so it is just a matter of convenience to choose separate or combined supplements.
Unfortunately, I have not seen any algae oil at a significantly different price, so the only cheaper solution is fish oil, at an 8-times lower price.
Hardly or just in small quantities?
With flaxseed oil, you could just consume more of it in order to offset the low ALA/DHA conversion rate. Or is there a downside to the idea?
Eating too much ALA, with the hope that this will be enough to ensure the DHA & EPA that you need, even in the worst case, can be harmful.
Nevertheless, this is a problem mostly for those who eat little or no animal foods. Otherwise you might get enough DHA and EPA from the normal food. Actually the best source of omega-3 acids are the animal brains, which are very tasty when well cooked.
However, since the Mad Cow disease, the popularity of eating brains has strongly declined, even among non-vegans.
That is why I'm curious. I've been living nearly-vegan for 15 years now and I've been "supplementing" with flaxseed oil for a few years now as well (still can't get used to the taste).