I believe all of those sources ship around the world as Dan will ship his seahorse products, not seahorses though, out of the US.
You can feed other products but first just check content to be sure any enrichment has a high DHA profile for best results.
While I personally don't like emulsions due to their limited storage capability, many people use them, and, Selco has a high DHA product that while not as high in DHA as Algamac 3050 I have no doubt it will suffice for the artemia being fed to seahorses.
http://inveaquaculture.com/product/a1-dha-selco/
Artemia will gather ANY particulate matter that is appropriately sized so it's a matter of being sure to particularize whatever you are using so the artemia appendages can capture those particles and pass them to their mouth/digestive tract. Contrary to what people think, they DON'T siphon the particles out of water that passes through them.
I don't know offhand if there are any fish foods or plankton that have sufficient DHA to use for enrichment. I do know that to use phytoplankton one need multiple species to get a proper balance of nutrition as there isn't a one fit's all type.
I use the powdered spirulina with added Algamac 3050 as it was the cheapest and most nutritious way I could feed and enrich my artemia.