Newark airport (EWR) is the closest airport to Spring Lake (~45min drive). Other airports you can look into are in Philadelphia (PHL) and New York (LGA). Both of these airports are ~1.5hr drive from Spring Lake.
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Spring Lake and surrounding towns all have train stations which are connected to Newark Airport. The most convenient way to navigate NJ transportation is downloading their app, NJ Transit. There, you can buy train tickets and see train routes/connections. There's usually a connection (commonly in Long Branch) while traveling to/from Newark Airport and Spring Lake.