Unfortunately, most LFSs keep the seahorses in tanks connected to a system that also includes other fish in other tanks. This exposes the seahorses to pathogens that they haven't grown up with and seahorses are very susceptible to those pathogens and often end up dying.
Also, many stores won't tell you who the breeder is, or, in many cases don't even know because they are buying from a trans shipper, and these often come from offshore sites with poor track record of survival.
Another problem is that most stores and their staff do not know how to properly keep seahorses for best chances of survival, even though they think they do.
Your best chances of success will come from buying from an established local hobbyist who is into breeding, or, to buy from a commercial breeder with established credentials in the hobby.
IMO, you can't find a better source for seahorses in the US than seahorsesource.com.