You can buy flood insurance through the National Flood Insurance Program (NFIP) or through a private insurer. Most flood insurance policies currently held are through NFIP. You can buy this federal flood insurance through non-federal, independent insurance agents.
Private insurance prices vary a lot between insurers, but they tend to be more expensive than NFIP. As always, make sure to shop around different private insurers and compare to an NFIP policy to make sure you find the best deal.