Maybe it's different in Europe, but I've never seen them in any fish shop here in the U.S., nor do I know of anyone who keeps them. I think the majority opinion among experienced fishkeepers is that they aren't meant for aquariums. I know there are markets for them
somewhere, and they are exploited for their "cute" factor. The overwhelming majority that are sold for aquarium use live short, miserable lives and die in the care of people who haven't anywhere near the knowledge or skill to care for them. Seahorses are not for the common aquarist, if they are for anyone at all.
I don't mean to sound cruel, but them's the facts.
pendragon!
Edit: If you are determined to keep them no matter what (which I personally advise against), at least please learn all you can about them first and do it right. It takes
a lot of skill, time, and expense to keep them. Just getting a proper setup on the front end will be very expensive, unless you have a properly established marine environment already.
Seahorse.Org looks like a credible site with lots of resources from experienced seahorse-keepers. Please do not go into it blindly and expect to learn it on the fly. If you must take ownership of such a delicate life, do it with responsibility and not recklessness.