Flood insurance is sold by private insurance companies. Flood insurance is not sold by the government. However, the federal government sets the terms and rates.
It's a good idea to buy your flood insurance from the firm that provides your homeowners' or your business' insurance. If your agent does not write flood insurance policies or you do not have an insurance agent, contact the National Flood Insurance Program (NFIP) at 1-888-379-9531 or www.floodsmart.gov.
Flood insurance can be purchased at any time. However, there is a 30-day waiting period before a flood insurance policy can become effective unless the policy is required by the lender at closing. If you are buying a house or other building that has flood insurance coverage, talk to your insurance agent about having the flood insurance policy "assigned" to you by the seller so that the coverage is continuous.