You can try this if you can get to Cancun or Nassau;

<U.S. residents, because of the embargo, find it difficult if not impossible to buy tickets to Havana without a Department of the Treasury license, and only licensed travel agencies may sell tickets to Cuba in the U.S. These agencies in turn may only sell tickets to licensed travellers.
Cubalinda has the solution to this problem. With us you can book your flight to Cuba on-line via Cancun, or Nassau, along with all the other services you need in Cuba such as hotels and rental cars.>