Both NIWA and Hawkes Bay Aquaculture ( Blair Grey) have developed methods to culture large numbers of sea horses for the aquarium market. I don't think HBA are selling any at the moment.
Here are a couple of scientific papers that may help:
Woods, C.M.C. (2000). Preliminary observations on breeding and rearing the seahorse Hippocampus abdominalis (Teleostei: Syngnathidae) in
captivity. New Zealand Journal of Marine and Freshwater Research
34: 475-485.
Woods, C.M.C. (2000). Improving initial survival in cultured seahorses,
Hippocampus abdominalis Leeson, 1827 (Teleostei: Syngnathidae).
Aquaculture 190: 377-388.
Woods, C. (2002). Seahorse culture. Seafood New Zealand, Vol. 10 No. 1 (Feb 2002).