another place to look is reefnutrition.com.
They sell direct to the consumer and also supply retailers. They’ve been in business at least 20 years, if not more.
I can’t vouch for the price or the quality of their product as I haven’t bought or tried their live plankton blend. I bought three bottles of algaebarn’s live blend on sale and I visually saw only about 10-25% of the plankton moving around under a microscope (I’m a total nerd). I’m not sure if the non-movers were dead or simple sessile species of plankton. I’m no biologist, but other than freshness, I don’t think it matters if plankton is alive or dead. I don’t think filter feeders are selective at all other than for the size of the particle they are eating.
Next time I purchase plankton from algaebarn, it will be when they aren’t running a sale. I don’t have anything against reef nutrition (AKA Reed Mariculture), I just tend to never switch to different products unless I have a good reason to. I’m weird like that.