Does seachem phytoplankton seem like a good source? That’s the only one I could get today, but I could get OceanMagik phytoplankton in a few days. I want to get the OceanMagik, but I’m worried the bivalve will starve. It’s been without food for 3 days.It does grow in the tank naturally higher light more phyto ... but it isn't really enough.
You can culture it, search for posts (possibly articles) here on R2R. You can buy it in a bottle. DT's used to be the brand back in the day. Or if you are lucky (like I am) your LFS carries it. It is basically green water.
Be warned it is a nutrient source and it can cause algae outbreaks. On my high light SPS tank, the tank will consume huge amounts of phyto and laugh. On my new tank that is currently essentially a FOWLR the low light and plenum help keep the algae outbreaks mild. I have a dusting of brown algae and the hungry limpets love the algae so it doesn't get very far. You can see my new tank on my build thread.
You do need to try to get some in to your tank but be judicial about it's use.