Other consumables from meat with notable effects here are carnosine and taurine. Carnosine in particular helps with janitorial processes that happen during sleep.
After switching to a vegan diet and after some experimentation with other variants, I have eventually settled on adding to the food small amounts of creatine monohydrate, taurine and choline bitartrate in pure powder form, exactly like you would add table salt.
For me it has worked fine.
However, among the nutritional supplements for a vegan diet I make a temporary exception for cod liver oil (or other fish oil). Even if there is an adequate vegan substitute (Schizochytrium oil, which is produced by a cultivated fungus-like organism, despite being marketed as "algae" oil), for now it is still too expensive. Its price has been slowly decreasing, so I expect that in a few years its price will become no greater than double in comparison with fish oil, when I would consider switching to it as acceptable.
I'm assuming the cod liver oil is for the Omega content?
I'm fortunate to have access to some decent Omega supplements, but bivalves are also excellent sources of Omegas (in addition to Taurine, Choline, and Creatine). If your vegan diet is motivated by vegan or environmental ethics, I'm curious why you've landed on fish oil rather than bivalves (since fish have brains and I believe oyster farming specifically is supposed to also have a positive impact on the habitats it's performed in)?
At least where I live, in Europe, fish oil is many times less expensive than bivalves, which are priced like luxury food.
Moreover, fish oil is much more convenient, because after I buy a liter, that is enough for more than 3 months, so I have to take care about this only very infrequently.
Bivalves can be stored only frozen and even in that case they occupy a huge volume in comparison with the equivalent amount of fish oil. They would also require additional time each day for preparation and cooking (even if that takes only a few minutes when done in a microwave oven). With cod liver oil or other fish oil, I just add a minute amount of it to the vegetable oil that I add to food anyway, so the extra time for it is just taking the bottle from the refrigerator and putting it back, which adds perhaps ten seconds to the cooking time.
When well prepared, bivalves are very tasty and healthy but they are not an optimal solution for ensuring an adequate daily intake of essential fatty acids, except perhaps in places on marine coasts, close to some farming site, where there might be possible to have continuous access to a guaranteed supply of cheap fresh bivalves.
I would rather not use fish oil, so as I have said, I am waiting for the production process for Schizochytrium oil to become more efficient, so that the premium price over fish oil will decrease enough. When I have tried for the first time Schizochytrium oil, a few years ago, it was at least eight times more expensive than fish oil. The last time when I have checked, it could be found (when searching enough) for as little as three times the price of fish oil (per its DHA+EPA content), so there is hope for a further price reduction.
Good point, I've only started consuming bivalves recently (having never purchased them in my life before) and live on a marine coastal city where bivalves naturally wash up on shore in addition to being farmed, so I didn't consider that the prices I see would be significantly different than a non-coastal place, especially since I also often purchase mussels, clams, and oysters which are harvested in other countries and shipped here.
For me, I may spend the same amount on bivalves as I would on an equivalent (by protein content) amount of mock meat products or veggie burgers. The bivalves are no doubt much healthier, but have fewer calories which is either a good or bad thing depending on what you're trying to optimize for.
