You will need an e-ticket for the Dominican Republic. This is the official website https://eticket.migracion.gob.do/Home/

Other sites charge a fee (Thanks AMEX fraud alert!). I recommend the free version!

For up to date travel info check out the link below:

https://travel.state.gov/content/travel/en/international-travel/International-Travel-Country-Information-Pages/DominicanRepublic.html