Yes, there is legitimate wedding insurance, but I'd make sure you stick with the bigger well known insurance companies, or completely investigate the payout history of a smaller company. Its not terribly expensive, usually under $100. They don't compensate for inconvenience, only for actual monetary losses.
So if for instance flowers show up late, no payment. But if the florist cancels at the last minute, and you have to put in a rush order (more expensive) with someone else, they'll cover that.
Usually the thing that goes wrong with weddings is weather related, so make sure to plan for an indoor wedding and reception. Or if you have outdoor plans, make sure the place putting it on has a backup plan for bad weather.