The closest airport to Managaha Beach is I Fadang Airport, which is located in Garapan. If you plan on visiting or staying in this area, landing at I Fadang Airport will be your best bet.
If you are looking to fly to Northern Mariana Islands for less, look to airlines like T'way Air, Jeju Air, and United Airlines, with prices starting at $107, $169, and $175 respectively.
The Northern Mariana Islands has not restricted entry to travelers, but travelers must complete a Declaration Form online at www.governor.gov.mp/covid-19/travel at least 3 days before arrival. Residents of the Northern Mariana Islands are subject to self-quarantine for 14 days, while foreign visitors will be subject to government quarantine, unless they have a negative PCR test for COVID-19 issued within 3-6 days of their arrival. The certificate must be in English. Travelers arriving without a proof of COVID-19 PCR test are subject to quarantine for 5 days. If you are planning to travel to Northern Mariana Islands at this time, it is recommended that you stay up to date on current restrictions and follow proper safety measures while in public.