Insurance for Visitors or Residents?
If you are visiting get it before you go (though HMO's or insurance from work may be adequate out of country for shorter periods) , if you live there, search on the term and there are quite a few posts on the topic already.
If you are an ex-pat (living in DR, not a DR citzien) looking for quite comprehensive coverage (e.g. $$$ coverage, evacuation, etc.) you can search via Google ( which nobody seems to try first for any question anymore) and there are numerous firms that sell to you on the basis you are not in your country of residence. Quite a few are based in the UK.